ipfilter: Add sysctl flag CTLFLAG_TUN to loader tunable

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.

Reviewed by: cy (for network)
Fixes: a805ffbcbce8 ipfilter: Make LARGE_NAT a tunable
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42005

(cherry picked from commit ba883e7a5ac43b27a373f2b1b0044fd2eb769d4e)