etc: mask zfs-load-key.service

Otherwise, systemd-sysv-generator will generate a service equivalent
that breaks the boot: under systemd this is covered by
zfs-mount-generator

We already do this for zfs-import.service, and other init scripts are
suppressed automatically by the "actual" .service files

Fixes: commit f04b97620059d08b37d9e80ada397e742bb2f311 ("Add init script
to load keys")
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #14010
Closes #14019