MFV r354383: 10592 misc. metaslab and vdev related ZoL bug fixes

MFV r354383: 10592 misc. metaslab and vdev related ZoL bug fixes

illumos/illumos-gate@555d674d5d4b8191dc83723188349d28278b2431
https://github.com/illumos/illumos-gate/commit/555d674d5d4b8191dc83723188349d28278b2431

https://www.illumos.org/issues/10592
  This is a collection of recent fixes from ZoL:
  8eef997679b Error path in metaslab_load_impl() forgets to drop ms_sync_lock
  928e8ad47d3 Introduce auxiliary metaslab histograms
  425d3237ee8 Get rid of space_map_update() for ms_synced_length
  6c926f426a2 Simplify log vdev removal code
  21e7cf5da89 zdb -L should skip leak detection altogether
  df72b8bebe0 Rename range_tree_verify to range_tree_verify_not_present
  75058f33034 Remove unused vdev_t fields

Portions contributed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Author: Serapheim Dimitropoulos <serapheim@delphix.com>
MFC after:	4 weeks