devres: Add devm_get_free_pages API

devm_get_free_pages() and devm_free_pages() are the managed counterparts
for __get_free_pages() and free_pages().

Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>