illumos 4088 - use after free in arc_release()

4088 use after free in arc_release()
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Garrett D'Amore <garrett@damore.org>
Reviewed by: Saso Kiselkov <skiselkov.ml@gmail.com>
Approved by: Dan McDonald <danmcd@nexenta.com>
Ported by: Tim Chase <tim@chase2k.com>

From the illumos issue:

A race-induced use after free occurs in arc_release() where the ARC header
is used outside the critical section protected by the hash_lock.