Fix a race between the syncer and umount. When you umount a softupdates

Fix a race between the syncer and umount.  When you umount a softupdates
filesystem softdep_process_worklist() is called in a loop until it indicates
that no dependancies remain, but the determination of that fact depends on
there only being one softdep_process_worklist() instance running.  It was
possible for the syncer to also be running softdep_process_worklist()
and the pre-existing checks in the code to prevent this were not sufficient
to prevent the race.  This patch solves the problem.

Approved-by: mckusick