nvmet: add metadata/T10-PI support

Expose the namespace metadata format when PI is enabled. The user needs
to enable the capability per subsystem and per port. The other metadata
properties are taken from the namespace/bdev.

Usage example:
echo 1 > /config/nvmet/subsystems/${NAME}/attr_pi_enable
echo 1 > /config/nvmet/ports/${PORT_NUM}/param_pi_enable

Signed-off-by: Israel Rukshin <israelr@mellanox.com>
Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: James Smart <james.smart@broadcom.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>