cryptsetup-generator: do not bind to the decrypted device unit (#6538)
This breaks things when the decrypted device is not immediately
`SYSTEMD_READY=1` (e. g. when a multi-device btrfs system is placed on
multiple cryptsetup devices).
Fixes #6537.
(cherry picked from commit e9ea4526)
(cherry picked from commit f5f367d4)