[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>
diff --git a/include/linux/if_ether.h b/include/linux/if_ether.h
index 42dc6a3..e157c13 100644
--- a/include/linux/if_ether.h
+++ b/include/linux/if_ether.h
@@ -129,7 +129,8 @@
 /*
  *	Display a 6 byte device address (MAC) in a readable format.
  */
-extern __pure char *print_mac(char *buf, const unsigned char *addr);
+extern char *print_mac(char *buf, const unsigned char *addr);
+#define MAC_FMT "%02x:%02x:%02x:%02x:%02x:%02x"
 #define MAC_BUF_SIZE	18
 #define DECLARE_MAC_BUF(var) char var[MAC_BUF_SIZE] __maybe_unused