[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:

235365f3aaaa10b7056293877c0ead50425f25c7 ("net/8021q/vlan_dev.c: Use
print_mac."), fea5fa875eb235dc186b1f5184eb36abc63e26cc ("[NET]: Remove
MAC_FMT"), and 8f789c48448aed74fe1c07af76de8f04adacec7d ("[NET]:
Elminate spurious print_mac() calls.")

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