staging/xgifb: Add mutext for fb_mmap locking

This adds a mutex for fb_mmap around smem_start and smem_len
so the mutex inside the fb_mmap() is actually used.  Changing of
these fields before calling the framebuffer_register() are not mutexed.

We check whether framebuffer_register has been called by reading
fbinfo->count.

See 537a1bf0 - "fbdev: add mutex for fb_mmap locking" by Krzysztof Helt
for details.
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/xgifb/XGI_main_26.c

@@ -1350,10 +1350,17 @@ static int XGIfb_get_fix(struct fb_fix_screeninfo *fix, int con,
 
 	strncpy(fix->id, "XGI", sizeof(fix->id) - 1);
 
-	fix->smem_start = xgifb_info->video_base;
+	/* if register_framebuffer has been called, we must lock */
+	if (atomic_read(&info->count))
+		mutex_lock(&info->mm_lock);
 
+	fix->smem_start = xgifb_info->video_base;
 	fix->smem_len = xgifb_info->video_size;
 
+	/* if register_framebuffer has been called, we can unlock */
+	if (atomic_read(&info->count))
+		mutex_unlock(&info->mm_lock);
+
 	fix->type = FB_TYPE_PACKED_PIXELS;
 	fix->type_aux = 0;
 	if (xgifb_info->video_bpp == 8)