io_uring: add a helper for putting rsrc nodes

Add a simple helper to encapsulating dropping rsrc nodes references,
it's cleaner and will help if we'd change rsrc refcounting or play with
percpu_ref_put() [no]inlining.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/63fdd953ac75898734cd50e8f69e95e6664f46fe.1650311386.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>