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

This patch fixes the following warning:

drivers/xen/xen-pciback/xenbus.c: In function ‘xen_pcibk_frontend_changed’:
drivers/xen/xen-pciback/xenbus.c:545:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (xenbus_dev_is_online(xdev))
      ^
drivers/xen/xen-pciback/xenbus.c:548:2: note: here
  case XenbusStateUnknown:
  ^~~~

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>
Signed-off-by: Juergen Gross <jgross@suse.com>