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>