sock: expose so_timestamp options for mptcp

This exports SO_TIMESTAMP_* function for re-use by MPTCP.

Without this there is too much copy & paste needed to support
this from mptcp setsockopt path.

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>