lagg(4): Do not enter net epoch recursively

This saves a little resources.

No functional change intended.

Reviewed by: kp
Fixes: b8a6e03fac92 Widen NET_EPOCH coverage
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39267

(cherry picked from commit d4a80d21b3d32a2de02d1820cc1f38dba1f127cb)