Key inheritance fixes for ZVOL, root dataset encryption fixes. 

"zpool create -O encryption=on" would not set dataset properties correctly
nor load key.

ZVOL key inheritance (under an encrypted ZFS dataset) would pick wrong
cipher (use parent cipher instead of zvol's aes-128-ctr).

ZVOLs from key inheritance would not call zvol_create_minor() to populate
/dev/zd* nodes.