Use vnlru_free(9) to implement dnlc_reduce_cache().

This apparently puts ARC back under the limits after the vnode pressure
rework in r291244, in particular due to the kmem exhaustion.

Based on patch by: mckusick
Reviewed by: avg, mckusick
Tested by: allanjude, madpilot
Sponsored by: The FreeBSD Foundation
Approved by: re (gjb)