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.

Reviewed By: gallatin, melifaro
Differential Revision: https://reviews.freebsd.org/D28377

(cherry picked from commit 24a8f6d369962f189ad808f538029179b1e7dc2f)