9p: Skip check for mandatory locks when unlocking

While investigating a bug, I came across a possible bug in v9fs. The
problem is similar to the one reported for NFS by ASANO Masahiro in
http://lkml.org/lkml/2005/12/21/334.

v9fs_file_lock() will skip locks on file which has mode set to 02666.
This is a problem in cases where the mode of the file is changed after
a process has obtained a lock on the file. Such a lock will be skipped
during unlock and the machine will end up with a BUG in
locks_remove_flock().

v9fs_file_lock() should skip the check for mandatory locks when
unlocking a file.
Signed-off-by: NSachin Prabhu <sprabhu@redhat.com>
Signed-off-by: NEric Van Hensbergen <ericvh@gmail.com>

fs/9p/vfs_file.c

@@ -114,7 +114,7 @@ static int v9fs_file_lock(struct file *filp, int cmd, struct file_lock *fl)
 	P9_DPRINTK(P9_DEBUG_VFS, "filp: %p lock: %p\n", filp, fl);
 
 	/* No mandatory locks */
-	if (__mandatory_lock(inode))
+	if (__mandatory_lock(inode) && fl->fl_type != F_UNLCK)
 		return -ENOLCK;
 
 	if ((IS_SETLK(cmd) || IS_SETLKW(cmd)) && fl->fl_type != F_UNLCK) {