sctp: also copy sk_tsflags when copying the socket

As we are keeping timestamps on when copying the socket, we also have to
copy sk_tsflags.

This is needed since b9f40e21 ("net-timestamp: move timestamp flags
out of sk_flags").

Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>