MFV r353630: 10809 Performance optimization of AVL tree comparator functions

MFV r353630: 10809 Performance optimization of AVL tree comparator functions

illumos/illumos-gate@c4ab0d3f46036e85ad0700125c5a83cc139f55a3
https://github.com/illumos/illumos-gate/commit/c4ab0d3f46036e85ad0700125c5a83cc139f55a3

https://www.illumos.org/issues/10809
  Port ZoL ee36c709c3d Performance optimization of AVL tree comparator functions

This is a followup to r337567 that imported the ZoL commit directly into
FreeBSD.  It seems that at the time we did not have some of the earlier
changes, so some pieces of the ZoL change were not applicable.  Also,
the illumos version got a few style cleanups.  Some changes were missed
or incorrectly merged (e.g., vdev_cache_lastused_compare and
metaslab_rangesize_compare).

Obtained from:	ZoL, illumos
MFC after:	25 days
X-MFC after:	r353634