[NET]: Elminate spurious print_mac() calls.

Patrick McHardy notes that print_mac() can get invoked
even if the result it unused (f.e. as an argument to
pr_debug() when DEBUG is not defined).

Mark this function as "__pure" to eliminate this problem.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/if_ether.h

@@ -129,7 +129,7 @@ extern ssize_t sysfs_format_mac(char *buf, const unsigned char *addr, int len);
 /*
  *	Display a 6 byte device address (MAC) in a readable format.
  */
-extern char *print_mac(char *buf, const unsigned char *addr);
+extern __pure char *print_mac(char *buf, const unsigned char *addr);
 #define MAC_BUF_SIZE	18
 #define DECLARE_MAC_BUF(var) char var[MAC_BUF_SIZE] __maybe_unused