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

fs/ufs/super.c

@@ -95,22 +95,18 @@
 
 void lock_ufs(struct super_block *sb)
 {
-#if defined(CONFIG_SMP) || defined (CONFIG_PREEMPT)
 	struct ufs_sb_info *sbi = UFS_SB(sb);
 
 	mutex_lock(&sbi->mutex);
 	sbi->mutex_owner = current;
-#endif
 }
 
 void unlock_ufs(struct super_block *sb)
 {
-#if defined(CONFIG_SMP) || defined (CONFIG_PREEMPT)
 	struct ufs_sb_info *sbi = UFS_SB(sb);
 
 	sbi->mutex_owner = NULL;
 	mutex_unlock(&sbi->mutex);
-#endif
 }
 
 static struct inode *ufs_nfs_get_inode(struct super_block *sb, u64 ino, u32 generation)