MFC r296530: MFV r296529:
6672 arc_reclaim_thread() should use gethrtime() instead of ddi_get_lbolt()
6673 want a macro to convert seconds to nanoseconds and vice-versa

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>
Author: Eli Rosenthal <eli.rosenthal@delphix.com>

illumos/illumos-gate@a8f6344fa0921599e1f4511e41b5f9a25c38c0f9

MFC r260234, r260328:
Remove extra conversion to nanoseconds from ddi_get_lbolt64().

As result this uses one multiplication and shifts instead of one division
and two multiplications.

MFC r260234, r260328:
Remove extra conversion to nanoseconds from ddi_get_lbolt64().

As result this uses one multiplication and shifts instead of one division
and two multiplications.