Add some global counters for SMR. These may eventually become per-smr
counters. In my stress test there is only one poll for every 15,000
frees. This means we are effectively amortizing the cache coherency
overhead even with very high write rates (3M/s/core).

Reviewed by: markj, rlibby
Differential Revision: https://reviews.freebsd.org/D23463