vm_pageout_scan_inactive: take a lock break

In vm_pageout_scan_inactive, release the object lock when we go to
refill the scan batch queue so that someone else has a chance to acquire
it. This improves access latency to the object when the pagedaemon is
processing many consecutive pages from a single object, and also in any
case avoids a hiccup during refill for the last touched object.

Reviewed by: alc, markj (previous version)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D45288

(cherry picked from commit a216e311a70cc87a5646f4306e36c60a51706699)