dummynet: Move packet counters into dn_cfg

Move the packets counters into the dn_cfg struct. This reduces the
global name space use for dummynet and will make future work for things
like vnet support and re-use in pf easier.

Reviewed by: donner
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29245