[SCTP]: Do not timestamp every SCTP packet.

We only need the timestamp on COOKIE-ECHO chunks, so instead of always
timestamping every SCTP packet, let common code timestamp if the socket
option is set.  For COOKIE-ECHO, simply get the time of day if we don't
have a timestamp.  This introduces a small possibility that the cookie
may be considered expired, but it will be renegotiated.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>