nvme: fix and clarify the check for missing metadata

Update the check in nvme_setup_rw for missing metadata so that it is
together with the other metadata handling, does not contain impossible
to reach conditions and warns if we get an impossible requests for
a (non-PI) metadata-enabled namespace when CONFIG_BLK_DEV_INTEGRITY
is not set.

Also add a little helper that checks if a given metadata configuration
contains protection information

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Javier González <jg@lightnvm.io>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>