Illumos 5445 - Add more visibility via arcstats

5445 Add more visibility via arcstats; specifically arc_state_t
stats and differentiate between "data" and "metadata"
Reviewed by: Basil Crow <basil.crow@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Bayard Bell <bayard.bell@nexenta.com>
Approved by: Robert Mustacchi <rm@joyent.com>

References:

https://www.illumos.org/issues/5445
https://github.com/illumos/illumos-gate/commit/4076b1b

Porting Notes:

This patch is an improved version of cc7f677 which was previously
merged in ZoL. This patch incorporates the additional improvements
which were made upstream.

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #3533