net: mctp: unshare packets when reassembling

Ensure that the frag_list used for reassembly isn't shared with other
packets. This avoids incorrect reassembly when packets are cloned, and
prevents a memory leak due to circular references between fragments and
their skb_shared_info.

The upcoming MCTP-over-USB driver uses skb_clone which can trigger the
problem - other MCTP drivers don't share SKBs.

A kunit test is added to reproduce the issue.

Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Fixes: 4a992bbd ("mctp: Implement message fragmentation & reassembly")
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250306-matt-mctp-usb-v1-1-085502b3dd28@codeconstruct.com.au


Signed-off-by: Paolo Abeni <pabeni@redhat.com>