tcp: Add SNMP counter for DEFER_ACCEPT

Its currently hard to diagnose when ACK frames are dropped because an
application set TCP_DEFER_ACCEPT on its listening socket.

See http://bugzilla.kernel.org/show_bug.cgi?id=15507

This patch adds a SNMP value, named TCPDeferAcceptDrop

netstat -s | grep TCPDeferAcceptDrop
    TCPDeferAcceptDrop: 0

This counter is incremented every time we drop a pure ACK frame received
by a socket in SYN_RECV state because its SYNACK retrans count is lower
than defer_accept value.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/snmp.h

@@ -227,6 +227,7 @@ enum
 	LINUX_MIB_SACKSHIFTFALLBACK,
 	LINUX_MIB_TCPBACKLOGDROP,
 	LINUX_MIB_TCPMINTTLDROP, /* RFC 5082 */
+	LINUX_MIB_TCPDEFERACCEPTDROP,
 	__LINUX_MIB_MAX
 };