3d13f313c, Add force_uaccess_{begin,end} helpers for get_fs/set_fs

diff -Naur drbd-9.0.26~0+git.9114a038.orig/drbd/drbd-kernel-compat/tests/have_force_uaccess_begin.c drbd-9.0.26~0+git.9114a038/drbd/drbd-kernel-compat/tests/have_force_uaccess_begin.c
--- drbd-9.0.26~0+git.9114a038.orig/drbd/drbd-kernel-compat/tests/have_force_uaccess_begin.c	1970-01-01 08:00:00.000000000 +0800
+++ drbd-9.0.26~0+git.9114a038/drbd/drbd-kernel-compat/tests/have_force_uaccess_begin.c	2020-12-09 19:36:17.554021899 +0800
@@ -0,0 +1,7 @@
+/* 3d13f313c Add force_uaccess_{begin,end} helpers for get_fs/set_fs */
+#include <linux/uaccess.h>
+
+void dummy(void)
+{
+	force_uaccess_begin();
+}
diff -Naur drbd-9.0.26~0+git.9114a038.orig/drbd/drbd_transport_tcp.c drbd-9.0.26~0+git.9114a038/drbd/drbd_transport_tcp.c
--- drbd-9.0.26~0+git.9114a038.orig/drbd/drbd_transport_tcp.c	2020-12-09 19:35:57.493892623 +0800
+++ drbd-9.0.26~0+git.9114a038/drbd/drbd_transport_tcp.c	2020-12-09 19:40:44.863689731 +0800
@@ -1145,7 +1145,12 @@
 	struct drbd_tcp_transport *tcp_transport =
 		container_of(transport, struct drbd_tcp_transport, transport);
 	struct socket *socket = tcp_transport->stream[stream];
+#ifdef COMPAT_HAVE_FORCE_UACCESS_BEGIN
+	mm_segment_t oldfs = force_uaccess_begin();
+#else
 	mm_segment_t oldfs = get_fs();
+	set_fs(KERNEL_DS);
+#endif
 	int len = size;
 	int err = -EIO;
 
@@ -1154,7 +1159,6 @@
 
 	msg_flags |= MSG_NOSIGNAL;
 	dtt_update_congested(tcp_transport);
-	set_fs(KERNEL_DS);
 	do {
 		int sent;
 
@@ -1180,7 +1184,11 @@
 		 * and add that to the while() condition below.
 		 */
 	} while (len > 0 /* THINK && peer_device->repl_state[NOW] >= L_ESTABLISHED */);
+#ifdef COMPAT_HAVE_FORCE_UACCESS_BEGIN
+	force_uaccess_end(oldfs);
+#else
 	set_fs(oldfs);
+#endif
 	clear_bit(NET_CONGESTED, &tcp_transport->transport.flags);
 
 	if (len == 0)