MFV r272494:

Make space_map_truncate() always do space_map_reallocate(). Without
this, setting space_map_max_blksz would cause panic for existing pool,
as dmu_objset_set_blocksize would fail if the object have multiple blocks.

Illumos issues:

5164 space_map_max_blksz causes panic, does not work
5165 zdb fails assertion when run on pool with recently-enabled

spacemap_histogram feature

MFC after: 2 weeks