[SCTP]: Use snmp_fold_field instead of a homebrew analogue.

SCPT already depends in INET, so this doesn't create additional
dependencies.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>