lagg(4): Refactor out some lagg protocol input routines into a default one

Those input routines are identical.

Also inline two fast paths.

No functional change intended.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39251

(cherry picked from commit dbe86dd5de18fdf61e1300f6575e0f50785bf6b3)