nvme-pci: iod npages fits in s8

The largest allowed transfer is 4MB, which can use at most 1025 PRPs.
Each PRP is 8 bytes, so the maximum number of 4k nvme pages needed for
the iod_list is 3, which fits in an 's8' type.

While modifying this field, change the name to "nr_allocations" to
better represent that this is referring to the number of units allocated
from a dma_pool.

Also introduce a BUILD_BUG_ON to ensure we never accidently increase the
largest transfer limit beyond 127 chained prp lists.

Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>