[SCTP]: Fix err_hdr assignment in sctp_init_cause.

The subh->err_hdr should point to the error header, not the data.

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>