zvol: fix delayed update to block device ro entry

The change in the zvol readonly property does not update the block
device readonly entry until the first IO to the ZVOL. This patch
addresses the issue by updating the block device readonly property
from the set property IOCTL call.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #15409