sock: expose so_timestamping options for mptcp

Similar to previous patch: expose SO_TIMESTAMPING helper so we do not
have to copy & paste this into the mptcp core.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>