packet_buffer: reset packet buffer state in Clone
packet_buffer: reset packet buffer state in Clone

The reset may be defensive, however, the code as-is is missing a clone
of EgressRoute, and was previously also missing the should-not-be-set
PreserveObject. The reset provides clarity of intent and safety against
missing field assignments.

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor