Make ZVOL writes in device mode support IO_SYNC flag.

MFC after:	1 month
Improve cache control support, including DPO/FUA flags and the mode page.

At this moment it works only for files and ZVOLs in device mode since BIOs
have no respective respective cache control flags (DPO/FUA).

MFC after:	1 month
Sponsored by:	iXsystems, Inc.