RDMA/hns: Fix return value in hns_roce_map_mr_sg

As described in the ib_map_mr_sg function comment, it returns the number
of sg elements that were mapped to the memory region. However,
hns_roce_map_mr_sg returns the number of pages required for mapping the
DMA area. Fix it.

Fixes: 9b2cf76c

 ("RDMA/hns: Optimize PBL buffer allocation process")
Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Link: https://lore.kernel.org/r/20240411033851.2884771-1-shaozhengchao@huawei.com
Reviewed-by: Junxian Huang <huangjunxian6@hisilicon.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>