lagg(4): Correctly define some sysctl variables

939a050ad96c virtualized lagg(4), but the corresponding sysctl of some
virtualized global variables are not marked with CTLFLAG_VNET. A try to
operate on those variables via sysctl will effectively go to the 'master'
copies and the virtualized ones are not read or set accordingly. As a
side effect, on updating the 'master' copy, the virtualized global
variables of newly created vnets will have correct values.

PR: 270705
Reviewed by: kp
Fixes: 939a050ad96c Virtualize lagg(4) cloner
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D39467