nvme-rdma: use nr_phys_segments when map rq to sgl

Use blk_rq_nr_phys_segments() instead of blk_rq_payload_bytes() to check
if a command contains data to be mapped.  This fixes the case where
a struct request contains LBAs, but it has no payload, such as
Write Zeroes support.

Fixes: 6e02318e ("nvme: add support for the Write Zeroes command")
Reported-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Tested-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>