io_uring: store rsrc node in req instead of refs

req->fixed_rsrc_refs keeps a pointer to rsrc node pcpu references, but
it's more natural just to store rsrc node directly. There were some
reasons for that in the past but not anymore.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/cee1c86ec9023f3e4f6ce8940d58c017ef8782f4.1650311386.git.asml.silence@gmail.com


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