nvme: introduce NVME_INLINE_METADATA_SG_CNT

SGL size of metadata is usually small. Thus, 1 inline sg should cover
most cases. The macro will be used for pre-allocate a single SGL entry
for metadata. The preallocation of small inline SGLs depends on SG_CHAIN
capability so if the ARCH doesn't support SG_CHAIN, use the runtime
allocation for the SGL. This patch is a preparation for adding metadata
(T10-PI) over fabric support.

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