dracut/zfs-load-key.sh: simplify import loop, quote variable assignments

The loop now has a less confusing condition and properly uses
systemctl(1) is-failed's return code instead of that entire mess

The assignments could turn into "var=val program" if encryptionroot
or keylocation had whitespace in them

As a bonus, this (mostly) silences shellcheck

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11198