Revert "ipv6: defrag: drop non-last frags smaller than min mtu"

This reverts commit 0ed4229b08c13c84a3c301a08defdc9e7f4467e6.
The change drops the non-last fragments whose size is less
than min mtu, this is currently breaking MT call on IWLAN.

It is observed that the SIP_INVITE has fragments of size
less than min mtu, currently reverting the change to allow
MT call to go through.

Change-Id: I78a10186792ec6c9febe141899ddc8ba5b815342
Signed-off-by: Sharath Chandra Vurukala <sharathv@codeaurora.org>
Signed-off-by: Kaustubh Pandey <kapandey@codeaurora.org>