Illumos 5368 - ARC should cache more metadata

5368 ARC should cache more metadata
Reviewed by: Alex Reece <alex.reece@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Richard Elling <richard.elling@richardelling.com>
Approved by: Dan McDonald <danmcd@omniti.com>

References:

https://www.illumos.org/issues/5368
https://github.com/illumos/illumos-gate/commit/3a5286a

Porting Notes:

The vast majority of this patch was already merged in the context
of the 06358ea changes. This is just a small hunk which was missed.

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