io_uring/rw: don't mask in f_iocb_flags

A previous commit changed overwriting kiocb->ki_flags with
->f_iocb_flags with masking it in. This breaks for retry situations,
where we don't necessarily want to retain previously set flags, like
IOCB_NOWAIT.

The use case needs IOCB_HAS_METADATA to be persistent, but the change
makes all flags persistent, which is an issue. Add a request flag to
track whether the request has metadata or not, as that is persistent
across issues.

Fixes: 59a7d12a ("io_uring: introduce attributes for read/write and PI support")
Signed-off-by: Jens Axboe <axboe@kernel.dk>