io_uring: add end-of-bits marker and build time verify it

io_uring: add end-of-bits marker and build time verify it

Not easy to tell if we're going over the size of bits we can shove
in req->flags, so add an end-of-bits marker and a BUILD_BUG_ON()
check for it.

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