mptcp: setsockopt: convert to mptcp_setsockopt_sol_socket_timestamping()

Split timestamping handling into a new function
mptcp_setsockopt_sol_socket_timestamping().
This is preparation for extending SO_TIMESTAMPING
for PHC binding, since optval will no longer be
integer.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>