io_uring/msg: initialise msg request opcode

It's risky to have msg request opcode set to garbage, so at least
initialise it to nop. Later we might want to add a user inaccessible
opcode for such cases.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/9afe650fcb348414a4529d89f52eb8969ba06efd.1743190078.git.asml.silence@gmail.com


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