io_uring: check for valid register opcode earlier

[ Upstream commit 34319084 ]

We only check the register opcode value inside the restricted ring
section, move it into the main io_uring_register() function instead
and check it up front.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>