zvol_wait: Ignore locked zvols
"When an encrypted zvol is locked the zfs-volume-wait service does not
start. The /sbin/zvol_wait should not wait for links when the volume
has property keystatus=unavailable."
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Thanks: James Dingwall <james-launchpad@dingwall.me.uk>
Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes #10662