net: pkt: Add explicit flag to indicate packet is IP reassembled

The current logic to determine whether a packet is IP reassembled is
flawed, as it only worked in certain conditions (which was ok, as the
conditions were satisfied for the current use case, but now it's a
public function). Therefore, add an explicit flag that indicates whether
a packet is IP reassembled or not.

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