net: Verify L4 checksum unconditionally for reassembled packets

In case of reassembled IP packets, we cannot rely on checksum
offloading as the drivers/HW has no means to verify L4 checksum before
the fragment is reassembled. Therefore, for such packets, verify L4
checksum in the software unconditionally.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>