dracut: Fix race condition between load-key and import

zfs-load-key.sh is called by the dracut-pre-mount.service unit which has
no explicit 'After' dependency on zfs-import.target. That way it can be
that the pool has not yet been imported and the zfs-load-key.sh finishes
without ever seeing the relevant pool.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Lorenz Hüdepohl <dev@stellardeath.org>
Closes #11500