[SCTP]: Remove multiple levels of msecs to jiffies conversions.

The SCTP sysctl entries are displayed in milliseconds, but stored
internally in jiffies. This results in multiple levels of msecs to
jiffies conversion and as a result produces a truncation error. This
patch makes things consistent in that we store and display defaults
in milliseconds and only convert once for use by association.
This patch also adds some sane min/max values so that we don't go off
the deep end.

Signed-off-by: Vladislav Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>