OpenZFS 7504 - kmem_reap hangs spa_sync and administrative tasks

Authored by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tim Chase <tim@chase2k.com>
Ported-by: George Melikov <mail@gmelikov.ru>

OpenZFS-issue: https://www.illumos.org/issues/7504
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/405a5a0
Closes #5736