v4l2: call VIDIOC_REQBUFS with count = 0 in pool_finalize
v4l2: call VIDIOC_REQBUFS with count = 0 in pool_finalize

Without this the following sequence fails:

- set_caps()
  - object_stop() (does nothing)
  - set_format() -> VIDIOC_S_FMT
- set_config() -> VIDIOC_REQBUFS with count = N
- set_caps()
  - object_stop()
    - pool_finalize()
  - set_format() -> VIDIOC_S_FMT => EBUSY

Usually the pool is started after set_config(), in which case object_stop()
will result in a pool_stop and therefore VIDIOC_REQBUFS with count = 0 but
that is not guaranteed.
Also calling VIDIOC_REQBUFS with count = 0 in pool_finalize() if necessary
fixes this problem.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=701543

pexip/gst-plugins-good,lovebug356/gst-plugins-good,jhodapp/gst-plugins-good,sh0/gst-plugins-good,rawoul/gst-plugins-good,ikonst/gst-plugins-good,BigBrother-International/gst-plugins-good,cfoch/gst-plugins-good,Kurento/gst-plugins-good,chamois94/gst-plugins-good,StreamUtils/gst-plugins-good,chamois94/gst-plugins-good,sebras/gst-plugins-good,ndufresne/gst-plugins-good,froggatt/gst-plugins-good-m,jcaden/gst-plugins-good,jhodapp/gst-plugins-good,jpakkane/gstreamer-plugins-good,collects/gst-plugins-good,krieger-od/gst-plugins-good,jpakkane/gstreamer-plugins-good,vatavuserban/gst-plugins-good,vatavuserban/gst-plugins-good,cablelabs/gst-plugins-good,lovebug356/gst-plugins-good,vatavuserban/gst-plugins-good,hizukiayaka/gst-plugins-good,cfoch/gst-plugins-good,ariscop/gst-plugins-good,wkatsak/gst-plugins-good,ariscop/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,kittee/gst-plugins-good,vatavuserban/gst-plugins-good,BigBrother-International/gst-plugins-good,froggatt/gst-plugins-good-m,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,surround-io/gst-plugins-good,reynaldo-samsung/gst-plugins-good,Kurento/gst-plugins-good,pexip/gst-plugins-good,jcaden/gst-plugins-good,GrokImageCompression/gst-plugins-good,cablelabs/gst-plugins-good,sh0/gst-plugins-good,cablelabs/gst-plugins-good,strukturag/gst-plugins-good,veo-labs/gst-plugins-good,stfl/gst-plugins-good,sh0/gst-plugins-good,GStreamer/gst-plugins-good,chamois94/gst-plugins-good,jpakkane/gstreamer-plugins-good,GrokImageCompression/gst-plugins-good,rawoul/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,cfoch/gst-plugins-good,hizukiayaka/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,davibe/gst-plugins-good-1.0,pexip/gst-plugins-good,krieger-od/gst-plugins-good,Lachann/gst-plugins-good,krieger-od/gst-plugins-good,cfoch/gst-plugins-good,ndufresne/gst-plugins-good,BigBrother-International/gst-plugins-good,davibe/gst-plugins-good-1.0,davibe/gst-plugins-good-1.0,surround-io/gst-plugins-good,greg80303/gst-plugins-good,reynaldo-samsung/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,GStreamer/gst-plugins-good,lovebug356/gst-plugins-good,Lachann/gst-plugins-good,StreamUtils/gst-plugins-good,greg80303/gst-plugins-good,loshca/gst-plugins-good,ikonst/gst-plugins-good,strukturag/gst-plugins-good,shelsonjava/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,BigBrother-International/gst-plugins-good,kittee/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,loshca/gst-plugins-good,jcaden/gst-plugins-good,krieger-od/gst-plugins-good,ndufresne/gst-plugins-good,sebras/gst-plugins-good,pexip/gst-plugins-good,collects/gst-plugins-good,sebras/gst-plugins-good,pexip/gst-plugins-good,froggatt/gst-plugins-good-m,veo-labs/gst-plugins-good,surround-io/gst-plugins-good,strukturag/gst-plugins-good,shelsonjava/gst-plugins-good,loshca/gst-plugins-good,jpakkane/gstreamer-plugins-good,sh0/gst-plugins-good,ariscop/gst-plugins-good,reynaldo-samsung/gst-plugins-good,jhodapp/gst-plugins-good,jhodapp/gst-plugins-good,greg80303/gst-plugins-good,Lachann/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,stfl/gst-plugins-good,cablelabs/gst-plugins-good,Kurento/gst-plugins-good,reynaldo-samsung/gst-plugins-good,jcaden/gst-plugins-good,strukturag/gst-plugins-good,Kurento/gst-plugins-good,ikonst/gst-plugins-good,sebras/gst-plugins-good,stfl/gst-plugins-good,GStreamer/gst-plugins-good,Kurento/gst-plugins-good,kittee/gst-plugins-good,surround-io/gst-plugins-good,shelsonjava/gst-plugins-good,chamois94/gst-plugins-good,veo-labs/gst-plugins-good,kittee/gst-plugins-good,StreamUtils/gst-plugins-good,veo-labs/gst-plugins-good,wkatsak/gst-plugins-good,ikonst/gst-plugins-good,ndufresne/gst-plugins-good,lovebug356/gst-plugins-good,collects/gst-plugins-good,stfl/gst-plugins-good,rawoul/gst-plugins-good,rawoul/gst-plugins-good,davibe/gst-plugins-good-1.0,ijsf/OpenWebRTC-gst-plugins-good,hizukiayaka/gst-plugins-good,shelsonjava/gst-plugins-good,greg80303/gst-plugins-good,froggatt/gst-plugins-good-m,loshca/gst-plugins-good,ariscop/gst-plugins-good,Lachann/gst-plugins-good,wkatsak/gst-plugins-good,collects/gst-plugins-good,hizukiayaka/gst-plugins-good,wkatsak/gst-plugins-good