RDMA/hns: Fix NULL pointer derefernce in hns_roce_map_mr_sg()

ib_map_mr_sg() allows ULPs to specify NULL as the sg_offset argument.
The driver needs to check whether it is a NULL pointer before
dereferencing it.

Fixes: d387d4b5 ("RDMA/hns: Fix missing pagesize and alignment check in FRMR")
Signed-off-by: Junxian Huang <huangjunxian6@hisilicon.com>
Link: https://patch.msgid.link/20241108075743.2652258-3-huangjunxian6@hisilicon.com


Signed-off-by: Leon Romanovsky <leon@kernel.org>