[SCTP]: Bring MAX_BURST socket option into ietf API extension compliance

Brings max_burst socket option set/get into line with the latest ietf
socket extensions api draft, while maintaining backwards
compatibility.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>