resource: add release_child_resources

Useful for freeing a portion of the resource tree, e.g. when trying to
reallocate resources more efficiently.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>