ethernet: Fix logging of frame length

Both the mbuf length and the total packet length are signed.

While here, update a stall comment to reflect the current practice.

Reviewed by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42390

(cherry picked from commit e7102929bf4fea4bf22855d2d6031edf6c413608)