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>