RDMA/rxe: Remove unused page_offset member

RDMA/rxe: Remove unused page_offset member

In rxe_map_mr_sg(), the `page_offset` member of the `rxe_mr` struct
was initialized based on `ibmr.iova`, which will be updated inside
ib_sg_to_pages() later.

Consequently, the value assigned to `page_offset` was incorrect. However,
since `page_offset` was never utilized throughout the code, it can be safely
removed to clean up the codebase and avoid future confusion.

Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Link: https://patch.msgid.link/20260116032833.2574627-1-lizhijian@fujitsu.com
Reviewed-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Signed-off-by: Leon Romanovsky <leon@kernel.org>