xfs: unregister inode shrinker before freeing filesystem structures

Currently we don't remove the XFS mount from the shrinker list until
late in the unmount path. By this time, we have already torn down
the internals of the filesystem (e.g. the per-ag structures), and
hence if the shrinker is executed between the teardown and the
unregistering, the shrinker will get NULL per-ag structure pointers
and panic trying to dereference them.

Fix this by removing the xfs mount from the shrinker list before
tearing down it's internal structures.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>

fs/xfs/linux-2.6/xfs_super.c

@@ -1156,9 +1156,13 @@ xfs_fs_put_super(
 
 	XFS_bflush(mp->m_ddev_targp);
 
+	/*
+	 * Unregister the memory shrinker before we tear down the mount
+	 * structure so we don't have memory reclaim racing with us here.
+	 */
+	xfs_inode_shrinker_unregister(mp);
 	xfs_unmountfs(mp);
 	xfs_freesb(mp);
-	xfs_inode_shrinker_unregister(mp);
 	xfs_icsb_destroy_counters(mp);
 	xfs_close_devices(mp);
 	xfs_free_fsname(mp);