Fixed bitrot in the non-softdep case of ufs_dirremove():

Fixed bitrot in the non-softdep case of ufs_dirremove():
- restored async mount support.  The first entry in a block is still
  always written synchronously, although it probably shouldn't be in
  the async case.
- restored use of BWRITE() instead of bowrite() for the DOWHITEOUT
  case, although bowrite() is probably better.

Broken by:	merge of softdep changes (rev.1.22).
Found by:	lmbench2 delete-file benchmarks.