net/packet: clarify source of pr_*() messages

Add pr_fmt macro to spell out the source of messages in prefix.

Before this patch:

  packet size is too long (1543 > 1518)

With this patch:

  af_packet: packet size is too long (1543 > 1518)
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -46,6 +46,8 @@
  *					Copyright (C) 2011, <lokec@ccs.neu.edu>
  */
 
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
 #include <linux/ethtool.h>
 #include <linux/types.h>
 #include <linux/mm.h>