Avoid fanout group collisions with best effort
Avoid fanout group collisions with best effort

Running multiple instances of netstack in the same network namespace can
cause collisions when enabling packet fanout for fdbased endpoints. The
only bulletproof fix is to run in different network namespaces, but by
using `getpid()` instead of 0 as the fanout ID starting point we can
avoid collisions in the common case, particularly when
testing/experimenting.

Addresses #6124

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