ipfilter: Add sysctl flag CTLFLAG_TUN to loader tunable

[zlei]

Summary

The sysctl variable net.inet.ipf.large_nat is actually a loader
tunable. Add sysctl flag CTLFLAG_TUN to it so that sysctl -T will
report it correctly.

No functional change intended.

Fixes: a805ffbcbce8 ipfilter: Make LARGE_NAT a tunable
MFC after: 3 days

Test Plan

Set kernel env, load module and verify the sysctl report.

# kenv net.inet.ipf.large_nat=1
net.inet.ipf.large_nat="1"
# kldload ipl
# sysctl -T net.inet.ipf.large_nat
net.inet.ipf.large_nat: 1

[cy]

LGTM