Set zfs_arc_meta_strategy to metadata only

Set zfs_arc_meta_strategy to metadata only

The previous default of "balanced" appears to have caused pathological
behavior, including very poor performance and 100% CPU load in the
arc_reclaim_thread.

The symptoms appeared when the daily periodic run started.
With this change, the system--and the ARC in particular--behaved
normally during a manual daily periodic run.

From Mark Johnston:  The port of the balanced strategy is incomplete,
since arc_prune_async() is a no-op on FreeBSD.  (This also seems
to imply that r337653 is a no-op.)  After 12 is branched we can
port the remaining bits and consider changing the default back.

Submitted by:	markj (essentially)
Reviewed by:	markj
Approved by:	re (gjb)
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D17156