[IPV4]: Reassembly trim not clearing CHECKSUM_HW

This was found by inspection while looking for checksum problems
with the skge driver that sets CHECKSUM_HW. It did not fix the
problem, but it looks like it is needed.

If IP reassembly is trimming an overlapping fragment, it
should reset (or adjust) the hardware checksum flag on the skb.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>