nvme-fc: use blk_rq_nr_phys_segments

Without this deallocate won't work properly due to the mismatch
of the bio/request size and the actual payload size.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: James Smart <james.smart@broadcom.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>