Merge branch 'kstat'

This branch updates several of the zfs kstats to take advantage
of the improved raw kstat functionality.  In addition, two new
kstats and a script called dbufstat.py are introduced.

Updated+New Kstats
* dbufs        - Stats for all dbufs in the dbuf_hash
* <pool>/txgs  - Stats for the last N txgs synced to disk
* <pool>/reads - Stats for rhe last N reads issues by the ARC
* <pool>/dmu_tx_assign - Histogram of tx assign times

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>