ss: do not emit warn while dumping MPTCP on old kernels

ss: do not emit warn while dumping MPTCP on old kernels

Prior to this commit, running 'ss' on a kernel older than v5.9
bumps an error message:

RTNETLINK answers: Invalid argument

When asked to dump protocol number > 255 - that is: MPTCP - 'ss'
adds an INET_DIAG_REQ_PROTOCOL attribute, unsupported by the older
kernel.

Avoid the warning ignoring filter issues when INET_DIAG_REQ_PROTOCOL
is used.

Additionally older kernel end-up invoking tcpdiag_send(), which
in turn will try to dump DCCP socks. Bail early in such function,
as the kernel does not implement an MPTCPDIAG_GET request.

Reported-by: "Rantala, Tommi T. (Nokia - FI/Espoo)" <tommi.t.rantala@nokia.com>
Fixes: 9c3be2c0eee0 ("ss: mptcp: add msk diag interface support")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>