pflow: Turn `pflowstats' statistics counters into per-CPU counters to make them mpsafe.

The weird interactions around pflow_flows' and sc_gcounter' replaced
by simple `pflow_flows' increment. Since the flow sequence is the 32
bits integer, the `sc_gcounter' type replaced by the type of uint32_t.

Obtained from: OpenBSD
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D43116