sock_diag: add SK_MEMINFO_BACKLOG

Adding socket backlog len in INET_DIAG_SKMEMINFO is really useful to
diagnose various TCP problems.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/sock_diag.h

@@ -18,6 +18,7 @@ enum {
 	SK_MEMINFO_FWD_ALLOC,
 	SK_MEMINFO_WMEM_QUEUED,
 	SK_MEMINFO_OPTMEM,
+	SK_MEMINFO_BACKLOG,
 
 	SK_MEMINFO_VARS,
 };