OpenZFS 6672 - arc_reclaim_thread() should use gethrtime()

6672 arc_reclaim_thread() should use gethrtime() instead of ddi_get_lbolt()

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>
Reviewed by: Robert Mustacchi <rm@joyent.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Ported-by: David Quigley <dpquigl@davequigley.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/6672
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/571be5c
Closes #4600