io_uring: simplify io_rsrc_node_ref_zero

Replace queue_delayed_work() with mod_delayed_work() in
io_rsrc_node_ref_zero() as the later one can schedule a new work, and
cleanup it further for better readability.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/3b2b23e3a1ea4bbf789cd61815d33e05d9ff945e.1617287883.git.asml.silence@gmail.com


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