vfs: fix NULL pointer dereference in fsync_buffers_list()

Fix NULL pointer dereference in fsync_buffers_list() introduced by recent fix
of races in private_list handling.  Since bh->b_assoc_map has been cleared in
__remove_assoc_queue() we should really use original value stored in the
'mapping' variable.
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/buffer.c

@@ -835,7 +835,7 @@ static int fsync_buffers_list(spinlock_t *lock, struct list_head *list)
 		smp_mb();
 		if (buffer_dirty(bh)) {
 			list_add(&bh->b_assoc_buffers,
-				 &bh->b_assoc_map->private_list);
+				 &mapping->private_list);
 			bh->b_assoc_map = mapping;
 		}
 		spin_unlock(lock);