linux-3.6-rc1 supports SK_MEMINFO_BACKLOG with commit d594e987c6f54
(sock_diag: add SK_MEMINFO_BACKLOG)

ss command can display it if provided by the kernel.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Vijay Subramanian <subramanian.vijay@gmail.com>