Fix encryption logic in systemd mount generator

Fix encryption logic in systemd mount generator

Previously the generator would skip a dataset if it wasn't mountable by
'zfs mount -a' (legacy/none mountpoint, canmount off/noauto). This also
skipped the generation of key-load units for such datasets, breaking
the dependency handling for mountable child datasets.

Reviewed-by: Antonio Russo <antonio.e.russo@gmail.com>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: InsanePrawn <insane.prawny@gmail.com>
Closes #9611