[SCTP]: Verify some mandatory parameters.

Verify init_tag and a_rwnd mandatory parameters in INIT and
INIT-ACK chunks.

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>