proc: Keep /proc/self cached separately from /proc/pid
proc: Keep /proc/self cached separately from /proc/pid

When dumping tasks we do a lot of open_proc()-s and to
speed this up the /proc/pid directory is opened first
and the fd is kept cached. So next open_proc()-s do just
openat(cached_fd, name).

The thing is that we sometimes call open_proc(PROC_SELF)
in between and proc helpers cache the /proc/self too. As
the result we have a bunch of

  open(/proc/pid)
  close()
  open(/proc/self)
  close()

see-saw-s in the middle of dumping tasks.

To fix this we may cache the /proc/self separately from
the /proc/pid descriptor. This eliminates quite a lot
of pointless open-s and close-s.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

gonkulator/criu,svloyso/criu,tych0/criu,AuthenticEshkinKot/criu,rentzsch/criu,fbocharov/criu,ldu4/criu,wtf42/criu,wtf42/criu,KKoukiou/criu-remote,tych0/criu,biddyweb/criu,ldu4/criu,gablg1/criu,biddyweb/criu,rentzsch/criu,eabatalov/criu,KKoukiou/criu-remote,efiop/criu,LK4D4/criu,AuthenticEshkinKot/criu,LK4D4/criu,gonkulator/criu,gablg1/criu,gonkulator/criu,tych0/criu,efiop/criu,marcosnils/criu,gablg1/criu,biddyweb/criu,ldu4/criu,gonkulator/criu,gonkulator/criu,KKoukiou/criu-remote,marcosnils/criu,AuthenticEshkinKot/criu,LK4D4/criu,tych0/criu,svloyso/criu,rentzsch/criu,tych0/criu,eabatalov/criu,sdgdsffdsfff/criu,sdgdsffdsfff/criu,KKoukiou/criu-remote,fbocharov/criu,LK4D4/criu,gablg1/criu,eabatalov/criu,rentzsch/criu,sdgdsffdsfff/criu,biddyweb/criu,fbocharov/criu,biddyweb/criu,marcosnils/criu,wtf42/criu,ldu4/criu,wtf42/criu,efiop/criu,eabatalov/criu,AuthenticEshkinKot/criu,marcosnils/criu,ldu4/criu,KKoukiou/criu-remote,svloyso/criu,tych0/criu,LK4D4/criu,rentzsch/criu,AuthenticEshkinKot/criu,wtf42/criu,gablg1/criu,marcosnils/criu,AuthenticEshkinKot/criu,efiop/criu,KKoukiou/criu-remote,gonkulator/criu,sdgdsffdsfff/criu,efiop/criu,svloyso/criu,svloyso/criu,LK4D4/criu,rentzsch/criu,eabatalov/criu,svloyso/criu,gablg1/criu,eabatalov/criu,fbocharov/criu,fbocharov/criu,sdgdsffdsfff/criu,wtf42/criu,biddyweb/criu,sdgdsffdsfff/criu,fbocharov/criu,efiop/criu,marcosnils/criu,ldu4/criu