MFC r272884:

accept(2) may and can return EAGAIN, document it.
MFC r272504: 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