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