MFC r337970: 9738 Fix third block copy allocations, broken at 9112.

Use METASLAB_WEIGHT_CLAIM weight to allocate tertiary blocks.
Previous use of METASLAB_WEIGHT_SECONDARY for that caused errors
later on metaslab_activate_allocator() call, leading to massive
load of unneeded metaslabs and write freezes.

Reviewed by: Paul Dagnelie <pcd@delphix.com>