sctp: fix sack_timeout sysctl min and max types

sctp_sack_timeout is defined as int, but the sysctl's maxsize is set
to sizeof(long) and the min/max are defined as long.

Signed-off-by:  <jean-mickael.guerin@6wind.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>