MFV r337029:

MFV r337029:
9426 metaslab size can exceed offset addressable by spacemap

metaslab size can exceed offset addressable by spacemap. The vdev can
address up to 2^63 * SPA_MAXBLOCKSIZE (512). A metaslab can address up to
2^47 * 2^vdev_ashift. Therefore we may need to increase the number of
metaslabs so that the maximum metaslab size is capped at the amount that
can be addressed by the spacemap. This should happen in
vdev_metaslab_set_size().

illumos/illumos-gate@b4bf0cf0458759c67920a031021a9d96cd683cfe

Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Matt Ahrens <matt@delphix.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author:     Don Brady <don.brady@delphix.com>