MFC r286763: 5497 lock contention on arcs_mtx

MFC r286763: 5497 lock contention on arcs_mtx

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Richard Elling <richard.elling@richardelling.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Prakash Surya <prakash.surya@delphix.com>

illumos/illumos-gate@244781f10dcd82684fd8163c016540667842f203

This patch attempts to reduce lock contention on the current arc_state_t
mutexes. These mutexes are used liberally to protect the number of LRU
lists within the ARC (e.g. ARC_mru, ARC_mfu, etc). The granularity at
which these locks are acquired has been shown to greatly affect the
performance of highly concurrent, cached workloads.

git-svn-id: svn://svn.freebsd.org/base/stable/10@288581 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f