io_uring/net: extract io_send_select_buffer()

Extract a helper out of io_send() for provided buffer selection to
improve readability as it has grown to take too many lines.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>
Link: https://lore.kernel.org/r/26a769cdabd61af7f40c5d88a22469c5ad071796.1738087204.git.asml.silence@gmail.com


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