nvme: make nvme_ns_has_pi accessible to transports

Move the nvme_ns_has_pi() inline from core.c to the nvme.h header.
This allows use by the transports.

Signed-off-by: James Smart <jsmart2021@gmail.com>
[maxg: added a comment for nvme_ns_has_pi()]
Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Israel Rukshin <israelr@mellanox.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>