sctp: Various fixes for loader tunables
The following sysctl variables are actually loader tunables. Add sysctl
flag CTLFLAG_TUN to them so that sysctl -T will report them correctly.
- net.inet.sctp.tcbhashsize
- net.inet.sctp.pcbhashsize
- net.inet.sctp.chunkscale
The loader tunable 'net.inet.sctp.tcbhashsize' and 'net.inet.sctp.chunkscale'
are only used during vnet initializing, thus it make no senses to make them
writable tunable.
Validate the values of loader tunables on vnet initialize, reset them to
theirs defaults if invalid to prevent potential kernel panics.
Reviewed by: tuexen, transport, network
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42007
(cherry picked from commit dac91eb7660324677d8a2f71bd6f192422355ba1)