nvme-tcp: check sgl supported by target

SGLs support is mandatory for NVMe/tcp, make sure that the target is
aligned to the specification.

Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>