defrag: fix subsequent overlap of start of original (bsd)

Fix the BSD policy case where a subsequent fragment starts before an
original fragment and overlaps the beginning of the original
fragment. In this case the overlapping data from the new fragment is
preferred.

Suricata was preferring the data from the original fragment, but it
should only do that when the original fragment has an offset <= to the
new fragment.

- Adds test for this case

Bug: #6669
(cherry picked from commit f1709ea)