REQ_F_MUST_PUNT may seem looking good and clear, but it's the same
as not having REQ_F_NOWAIT set. That rather creates more confusion.
Moreover, it doesn't even affect any behaviour (e.g. see the patch
removing it from io_{read,write}).

Kill theg flag and update already outdated comments.
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>