ss: Support displaying and filtering on socket marks.

This allows the user to dump sockets with a given mark (via
"fwmark = 0x1234/0x1234" or "fwmark = 12345", etc.) , and to
display the socket marks of dumped sockets.

The relevant kernel commits are: d545caca827b ("net: inet: diag:
expose the socket mark to privileged processes.") and
- a52e95abf772 ("net: diag: allow socket bytecode filters to
match socket marks")

Signed-off-by: Lorenzo Colitti <lorenzo@google.com>