zfs extensible_dataset and large_blocks feature support

zfs extensible_dataset and large_blocks feature support

large blocks basically use extensible dataset feature, or to be exact,
setting recordsize above 128k will trigger large_block feature to be
enabled and storing such blocks is using feature extensible dataset. so
the extensible dataset is prerequisite.

Changes implement read support extensible dataset… instead of fixed DMU
types they dont specify type, making it possible to use fat zap objects
from bonus area.