MFV r275549:

Add a loader tunable, vfs.zfs.arc_meta_min, which controls how much metadata
ZFS should keep in ARC at minimum.

In arc_evict(), when doing recycle, take more factors into account by
applying the following policy:

  1. If no evictable data, evict metadata;
  2. If no evictable metadata, evict data;
  3. If we hit arc_meta_limit, evict metadata;
  4. If we haven't hit arc_meta_min, evict data;
  5* (Illumos only, not present in new FreeBSD code, yet) evict the oldest
     cached element from data and metadata.
     (FreeBSD) evict the data type specified by caller, which is the
     existing behavior.

Note that because of our splitted locks (implemented in r205231 to improve
scalability by reducing lock contention), implementing the fifth Illumos
behavior will not be cheap, so for now just implement the 1-4 and fall back
to current behavior for 5.

Illumos issue:
    5368 ARC should cache more metadata

MFC after:	2 months (assuming we didn't found better solution)
MFV r275550:

In addition to r273158, make the code in spa_sync() that checks if the
current TXG is a no-op TXG less fragile.

Illumos issue:
    5347 idle pool may run itself out of space

MFC after:	2 weeks