io_uring: extend io_uring_sqe flags bits

io_uring: extend io_uring_sqe flags bits

In hindsight everything is clearer, but it probably should've been known
that 8 bits of ->flags would run out sooner than later. Rather than
gobble up the last bit for a random use case, add a bit that controls
whether or not ->personality is used as a flags2 argument. If that is
the case, then there's a new IOSQE2_PERSONALITY flag that tells io_uring
which personality field to read.

While this isn't the prettiest, it does allow extending with 15 extra
flags, and retains being able to use personality with any kind of
command. The exception is uring cmd, where personality2 will overlap
with the space set aside for SQE128. If they really need that, then that
would have to be done via a uring cmd flag.

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