svcrdma: use offset_in_page() macro

Clean up: Use offset_in_page() macro instead of open-coding.
Reported-by: NGeliang Tang <geliangtang@gmail.com>
Signed-off-by: NChuck Lever <chuck.lever@oracle.com>
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

net/sunrpc/xprtrdma/svc_rdma_rw.c

@@ -372,8 +372,9 @@ static void svc_rdma_pagelist_to_sg(struct svc_rdma_write_info *info,
 	struct scatterlist *sg;
 	struct page **page;
 
-	page_off = (info->wi_next_off + xdr->page_base) & ~PAGE_MASK;
-	page_no = (info->wi_next_off + xdr->page_base) >> PAGE_SHIFT;
+	page_off = info->wi_next_off + xdr->page_base;
+	page_no = page_off >> PAGE_SHIFT;
+	page_off = offset_in_page(page_off);
 	page = xdr->pages + page_no;
 	info->wi_next_off += remaining;
 	sg = ctxt->rw_sg_table.sgl;