RDMA/siw: Introduce siw_get_page

Add the wrapper function to get either pbl page or umem page.

Acked-by: Bernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Link: https://lore.kernel.org/r/20231113115726.12762-2-guoqing.jiang@linux.dev


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