io_uring/rsrc: kill io_charge_rsrc_node()

It's only used from __io_req_set_rsrc_node(), and it takes both the ctx
and node itself, while never using the ctx. Just open-code the basic
refs++ in __io_req_set_rsrc_node() instead.

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