Fix net80211 fragment creation.

When creating fragment frames, the header length should honour the
DATAPAD flag.

This fixes the fragments that are queued to the ath(4) driver but it
doesn't yet fix fragment transmission.  That requires further changes
to the ath(4) transmit path.  Well, strictly speaking, it requires
further changes to _all_ wifi driver transmit paths, but this is at least
a start.

Tested:

* AR5416, STA mode, w/ fragthreshold set to 256.
Revert r250972 to fix build.