io_uring/rsrc: improve code generation for fixed file assignment

For the normal read/write path, we have already locked the ring
submission side when assigning the file. This causes branch
mispredictions when we then check and try and lock again in
io_req_set_rsrc_node(). As this is a very hot path, this matters.

Add a basic helper that already assumes we already have it locked,
and use that in io_file_get_fixed().

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