io_uring: check reserved fields for send/sendmsg

[ Upstream commit 588faa1e ]

We should check unused fields for non-zero and -EINVAL if they are set,
making it consistent with other opcodes.

Fixes: 0fa03c62 ("io_uring: add support for sendmsg()")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>