macvlan: 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/net/macvlan.c: In function ‘macvlan_do_ioctl’:
drivers/net/macvlan.c:839:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (!net_eq(dev_net(dev), &init_net))
      ^
drivers/net/macvlan.c:841:2: note: here
  case SIOCGHWTSTAMP:
  ^~~~

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 <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>