xfs: simplify /proc teardown & error handling

remove_proc_subtree() was added in 3.9, and can be
used to simplify our procfile creation error handling
and cleanup, removing the nested gotos.  It simply
removes fs/xfs and everything created under it.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>