io_uring/uring_cmd: use cached cmd_op in io_uring_cmd_sock()

io_uring_cmd_sock() does a normal read of cmd->sqe->cmd_op, where it
really should be using a READ_ONCE() as ->sqe may still be pointing to
the original SQE. Since the prep side already does this READ_ONCE() and
stores it locally, use that value rather than re-read it.

Fixes: 8e9fad0e ("io_uring: Add io_uring command support for sockets")
Link: https://lore.kernel.org/r/20250121-uring-sockcmd-fix-v1-1-add742802a29@google.com


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