sgi-xpc: mark expected switch fall-through

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

This patch fixes the following warning:

drivers/misc/sgi-xp/xpc_uv.c: In function ‘xpc_handle_activate_mq_msg_uv’:
drivers/misc/sgi-xp/xpc_uv.c:573:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   xpc_wakeup_channel_mgr(part);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/misc/sgi-xp/xpc_uv.c:575:2: note: here
  case XPC_ACTIVATE_MQ_MSG_MARK_ENGAGED_UV:
  ^~~~

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

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <[email protected]>
Acked-by: Robin Holt<[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

drivers/misc/sgi-xp/xpc_uv.c

@@ -572,6 +572,7 @@ xpc_handle_activate_mq_msg_uv(struct xpc_partition *part,
 
 		xpc_wakeup_channel_mgr(part);
 	}
+		/* fall through */
 	case XPC_ACTIVATE_MQ_MSG_MARK_ENGAGED_UV:
 		spin_lock_irqsave(&part_uv->flags_lock, irq_flags);
 		part_uv->flags |= XPC_P_ENGAGED_UV;