io_uring: reg buffer overflow checks hardening

We are safe with overflows in io_sqe_buffer_register() because it will
just yield alloc failure, but it's nicer to check explicitly.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/2b0625551be3d97b80a5fd21c8cd79dc1c91f0b5.1616624589.git.asml.silence@gmail.com


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