In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warning:

drivers/virt/vboxguest/vboxguest_core.c: In function ‘vbg_core_ioctl’:
drivers/virt/vboxguest/vboxguest_core.c:1486:10: warning: this statement may fall through [-Wimplicit-fallthrough=]
   f32bit = true;
   ~~~~~~~^~~~~~
drivers/virt/vboxguest/vboxguest_core.c:1489:2: note: here
  case VBG_IOCTL_HGCM_CALL(0):
  ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

Notice that, in this particular case, the code comment is modified
in accordance with what GCC is expecting to find.

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>