Fix Plymouth passphrase prompt in initramfs script

Entering the ZFS encryption passphrase under Plymouth wasn't working
because in the ZFS initrd script, Plymouth was calling zfs via
"--command", which wasn't passing through the filesystem argument to
zfs load-key properly (it was passing through the single quotes around
the filesystem name intended to handle spaces literally,
which zfs load-key couldn't understand).

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Garrett Fields <ghfields@gmail.com>
Signed-off-by: Richard Allen <belperite@gmail.com>
Issue #9193
Closes #9202