ss: add SK_MEMINFO_DROPS display

SK_MEMINFO_DROPS is added in linux-4.7 for TCP, UDP and SCTP

skmem will display the socket drop count using d prefix as in :

$ ss -tm src :22 | more
State      Recv-Q Send-Q Local Address:Port    Peer Address:Port
ESTAB      0      52     10.246.7.151:ssh      172.20.10.101:50759
	 skmem:(r0,rb8388608,t0,tb8388608,f1792,w2304,o0,bl0,d0)
Signed-off-by: Eric Dumazet <edumazet@google.com>

misc/ss.c

@@ -1905,6 +1905,10 @@ static void print_skmeminfo(struct rtattr *tb[], int attrtype)
 		(SK_MEMINFO_BACKLOG + 1) * sizeof(__u32))
 		printf(",bl%u", skmeminfo[SK_MEMINFO_BACKLOG]);
 
+	if (RTA_PAYLOAD(tb[attrtype]) >=
+		(SK_MEMINFO_DROPS + 1) * sizeof(__u32))
+		printf(",d%u", skmeminfo[SK_MEMINFO_DROPS]);
+
 	printf(")");
 }