ip_frag: fix fragmenting IPv4 fragment

ip_frag: fix fragmenting IPv4 fragment

Current implementation of rte_ipv4_fragment_packet() doesn’t take
into account offset and flag values of the given packet, but blindly
assumes they are always zero (original packet is not fragmented).
According to RFC791, fragment and flag values for new fragment
should take into account values provided in the original IPv4 packet.

Fixes: 4c38e5532a07 ("ip_frag: refactor IPv4 fragmentation into a proper library")
Cc: stable@dpdk.org
Signed-off-by: Huichao Cai <chcchc88@163.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>