MFV r277427: 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>
Author: Prakash Surya <prakash.surya@delphix.com>

illumos/illumos-gate@4076b1bf41cfd9f968a33ed54a7ae76d9e996fe8