sunrpc: set the bv_offset of first bvec in svc_tcp_sendmsg

svc_tcp_sendmsg used to factor in the xdr->page_base when sending pages,
but commit 5df5dd03 ("sunrpc: Use sendmsg(MSG_SPLICE_PAGES) rather
then sendpage") dropped that part of the handling. Fix it by setting
the bv_offset of the first bvec.

Fixes: 5df5dd03 ("sunrpc: Use sendmsg(MSG_SPLICE_PAGES) rather then sendpage")
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/svcsock.c

@@ -1244,6 +1244,9 @@ static int svc_tcp_sendmsg(struct socket *sock, struct xdr_buf *xdr,
 	if (ret != head->iov_len)
 		goto out;
 
+	if (xdr_buf_pagecount(xdr))
+		xdr->bvec[0].bv_offset = offset_in_page(xdr->page_base);
+
 	msg.msg_flags = MSG_SPLICE_PAGES;
 	iov_iter_bvec(&msg.msg_iter, ITER_SOURCE, xdr->bvec,
 		      xdr_buf_pagecount(xdr), xdr->page_len);