ip_mroute: Fix sysctl knobs

The loader tunable net.inet.ip.mfchashsize does not have corresponding
sysctl MIB entry. Just add it.

While here, the sysctl variable net.inet.pim.squelch_wholepkt is actually
a loader tunable. Add sysctl flag CTLFLAG_TUN to it so that sysctl -T
will report it correctly.

Reviewed by: kp
Fixes: 443fc3176dee Introduce a number of changes to the MROUTING code
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D41997