writeback: enforce s_umount locking in writeback_inodes_sb

Make sure that not only sync_filesystem but all callers of writeback_inodes_sb
have the superblock protected against remount.  As-is this disables all
functionality for these callers, but the next patch relies on this locking to
fix writeback_inodes_sb for sync_filesystem.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

fs/ubifs/budget.c

@@ -62,7 +62,9 @@
  */
 static void shrink_liability(struct ubifs_info *c, int nr_to_write)
 {
+	down_read(&c->vfs_sb->s_umount);
 	writeback_inodes_sb(c->vfs_sb);
+	up_read(&c->vfs_sb->s_umount);
 }
 
 /**