[NET]: Restore sanity wrt. print_mac().

MAC_FMT had only one user and we tried to get rid of
that, but this created more problems than it solved.

As a result, this reverts three commits:

235365f3 ("net/8021q/vlan_dev.c: Use
print_mac."), fea5fa87 ("[NET]: Remove
MAC_FMT"), and 8f789c48

 ("[NET]:
Elminate spurious print_mac() calls.")

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