MFC r353611: 10330 merge recent ZoL vdev and metaslab changes

MFC r353611: 10330 merge recent ZoL vdev and metaslab changes

illumos/illumos-gate@a0b03b161c4df3cfc54fbc741db09b3bdc23ffba
https://github.com/illumos/illumos-gate/commit/a0b03b161c4df3cfc54fbc741db09b3bdc23ffba

https://www.illumos.org/issues/10330
  3 recent ZoL changes in the vdev and metaslab code which we can pull over:
  PR 8324 c853f382db 8324 Change target size of metaslabs from 256GB to 16GB
  PR 8290 b194fab0fb 8290 Factor metaslab_load_wait() in metaslab_load()
  PR 8286 419ba59145 8286 Update vdev_is_spacemap_addressable() for new spacemap
  encoding

Author: Serapheim Dimitropoulos <serapheimd@gmail.com>
Obtained from:	illumos, ZoL
MFC after:	2 weeks