io_uring.h: restore correct sqe->user_data name

A sync-up commit inadvertently dropped a fix from commit:

369811b8c375 ("io_uring.h: fix user_data field name in comment")

while syncing other bits, let's restore it.

Fixes: 2fce7b7067a2 ("Sync kernel headers")
Signed-off-by: Jens Axboe <axboe@kernel.dk>