ip: compute checksums before fragmentation if offloaded

ip: compute checksums before fragmentation if offloaded

Type: fix

When a packet with checksum offloading enabled is
fragmented into multiple IP fragments, due to the
egress interface's MTU being smaller than the original
packet size, it gets dropped after reassembly because
the reconstructed packet contains an invalid checksum.

This patch addresses the issue by computing the checksum
in the IP fragmentation logic before the packet is
fragmented, ensuring the resulting fragments carry a
valid checksum upon reassembly.

Signed-off-by: Mohsin Kazmi <[email protected]>
Change-Id: I202f169887ae9594f9580a4c7901d7b4483ef5f9