Fix up alc_encap() / alc_start():
Fix up alc_encap() / alc_start():

- Remove unnecessary nsegs variable from alc_encap() and
  use map->dm_nsegs. Also remove unnecessary FreeBSD check
  for 0 DMA segments check.
- Remove printfs in alc_encap() failure paths that shouldn't
  be there.
- Add missing IF_PREPEND() from failure path coming off of
  alc_encap() within alc_start().
- Fix error handling within alc_encap(). Previously alc_encap()
  was attempting to unload a DMA map upon failure from
  bus_dmamap_load_mbuf() even though one wasn't loaded at that
  point and then always forcing mbufs through the EFBIG path.

Tested by Gabriel Linder.

From Brad

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars