MFC r262814

MFC r262814

- If we fail to do a non-blocking acquire of a buf lock while doing a
  waiting sync pass we need to do a blocking acquire and restart.
  Another thread, typically the buf daemon, may have this buf locked and
  if we don't wait we can fail to sync the file.  This lead to a great
  variety of softdep panics because we rely on all dependencies being
  flushed before proceeding in several cases.

Submitted by:	jeffr

git-svn-id: svn://svn.freebsd.org/base/stable/10@264491 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f