nvme: use ns->head->pi_size instead of t10_pi_tuple structure size

Currently kernel supports 8 byte and 16 byte protection information.
So, use ns->head->pi_size instead of sizeof(struct t10_pi_tuple).

Signed-off-by: Francis Pravin <francis.p@samsung.com>
Signed-off-by: Sathyavathi M <sathya.m@samsung.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>