Skip loading already loaded key

Don't ask for the password / try to load the key if the key for the 
encryptionroot is already loaded.  The user might have loaded the key 
manually or by other means before the scripts get called.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Witaut Bajaryn <vitaut.bayaryn@gmail.com>
Closes #9495
Closes #9529

etc/systemd/system-generators/zfs-mount-generator.in

@@ -182,6 +182,8 @@ process_line() {
         keyloadcmd="@sbindir@/zfs load-key '${dataset}'"
       elif [ "${p_keyloc}" = "prompt" ] ; then
         keyloadcmd="sh -c 'set -eu;"\
+"keystatus=\"\$\$(@sbindir@/zfs get -H -o value keystatus \"${dataset}\")\";"\
+"[ \"\$\$keystatus\" = \"unavailable\" ] || exit 0;"\
 "count=0;"\
 "while [ \$\$count -lt 3 ];do"\
 "  systemd-ask-password --id=\"zfs:${dataset}\""\