nvme-pci: remove redundant segment validation

We've validated the segment counts before calling nvme_map_data(),
so there is no need to validate again in nvme_pci_use_sgls(, which is
only called from nvme_map_data().

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>