hyperv_fb: Update screen_info after removing old framebuffer

On gen2 HyperV VM, hyperv_fb will remove the old framebuffer, and the
new allocated framebuffer address could be at a differnt location,
and it might be no longer a VGA framebuffer.

Update screen_info so that after kexec the kernel won't try to reuse
the old invalid/stale framebuffer address as VGA, corrupting memory.

[ mingo: Tidied up the changelog. ]
Signed-off-by: Kairui Song <kasong@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Jake Oshins <jakeo@microsoft.com>
Cc: Wei Hu <weh@microsoft.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Link: https://lore.kernel.org/r/20201014092429.1415040-3-kasong@redhat.com

drivers/video/fbdev/hyperv_fb.c

@@ -1114,8 +1114,15 @@ static int hvfb_getmem(struct hv_device *hdev, struct fb_info *info)
 getmem_done:
 	remove_conflicting_framebuffers(info->apertures,
 					KBUILD_MODNAME, false);
-	if (!gen2vm)
+
+	if (gen2vm) {
+		/* framebuffer is reallocated, clear screen_info to avoid misuse from kexec */
+		screen_info.lfb_size = 0;
+		screen_info.lfb_base = 0;
+		screen_info.orig_video_isVGA = 0;
+	} else {
 		pci_dev_put(pdev);
+	}
 	kfree(info->apertures);
 
 	return 0;