rtsock: Add sysctl flag CTLFLAG_TUN to loader tunable

The sysctl variable net.route.netisr_maxqlen 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: glebius
Approved by: re (gjb)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D41928

(cherry picked from commit 21a722d9593eb477f4ccdad011a2b4df0727fc62)
(cherry picked from commit bffb01eda9bffc3307ea8bf70f91087ed316e01c)