fs/ufs/super.c: fix potential race condition

fs/ufs/super.c: fix potential race condition

Let locking subsystem decide on mutex management.  As reported by Andrew
Morton this patch fixes a bug:

: lock_ufs() is assuming that on non-preempt uniprocessor, the calling
: code will run atomically up to the matching unlock_ufs().
:
: But that isn't true. The very first site I looked at (ufs_frag_map)
: does sb_bread() under lock_ufs().  And sb_bread() will call schedule(),
: very commonly.
:
: The ->mutex_owner stuff is a bit hacky but should work OK.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Cc: Evgeniy Dushistov <dushistov@mail.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>