io_uring: deduplicate SCM accounting

Merge io_sqe_file_register() and io_sqe_file_register(). The only
real difference left between them is from where we get an skb.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/dddda3039c71fcbec24b3465cbe8c7e7ae7bb0e8.1649334991.git.asml.silence@gmail.com


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