frag6: export another counter read-only by sysctl

Similar to the system global counter also export the per-VNET counter
"frag6_nfragpackets" detailing the current number of fragment packets
in this VNET's reassembly queues.
The read-only counter is helpful for in-VNET statistical monitoring and
for test-cases.

MFC after: 3 weeks
Sponsored by: Netflix