When we are about to send down to the driver layer
we need to make sure that the m_nextpkt field is NULL
else the lower layers may do unwanted things.

Approved by: re(gjb)
Reviewed By: gallatin, melifaro
Differential Revision: https://reviews.freebsd.org/D28377

(cherry picked from commit 538a81520b557f9964c5ebe912f9dff5996bed56)