net: pkt: Allow larger packets for IPv6 fragmentation

If the allocated net_pkt is larger than the network interface MTU,
then check if the IPv6 fragmentation is enabled and allow larger
net_pkt length as the IPv6 fragmentation will split the packet into
suitable parts.

Fixes #16354

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>