OpenZFS 7303 - dynamic metaslab selection

This change introduces a new weighting algorithm to improve
metaslab selection. The new weighting algorithm relies on the
SPACEMAP_HISTOGRAM feature. As a result, the metaslab weight
now encodes the type of weighting algorithm used (size-based
vs segment-based).

Porting Notes: The metaslab allocation tracing code is conditionally
removed on linux (dependent on mdb debugger).

Authored by: George Wilson <george.wilson@delphix.com>
Reviewed by: Alex Reece <alex@delphix.com>
Reviewed by: Chris Siden <christopher.siden@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <paul.dagnelie@delphix.com>
Reviewed by: Pavel Zakharov pavel.zakharov@delphix.com
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Don Brady <don.brady@intel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Don Brady <don.brady@intel.com>

OpenZFS-issue: https://www.illumos.org/issues/7303
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/d5190931bd
Closes #5404

module/zfs/spa_misc.c

@@ -1833,6 +1833,7 @@ spa_init(int mode)
 	refcount_init();
 	unique_init();
 	range_tree_init();
+	metaslab_alloc_trace_init();
 	ddt_init();
 	zio_init();
 	dmu_init();
@@ -1861,6 +1862,7 @@ spa_fini(void)
 	dmu_fini();
 	zio_fini();
 	ddt_fini();
+	metaslab_alloc_trace_fini();
 	range_tree_fini();
 	unique_fini();
 	refcount_fini();