sctp: fix a typo in prototype of __sctp_rcv_lookup()

Just to avoid confusion when people only reads this prototype.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>