io_uring: replace force_nonblock with flags

Replace bool force_nonblock with flags. It has a long standing goal of
differentiating context from which we execute. Currently we have some
subtle places where some invariants, like holding of uring_lock, are
subtly inferred.

Signed-off-by: Pavel Begunkov <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

Author: isilence