io_uring: name rsrc bits consistently

Keep resource related structs' and functions' naming consistent, in
particular use "io_rsrc" prefix for everything.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/962f5acdf810f3a62831e65da3932cde24f6d9df.1617287883.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>