Systemd mount generator: don't fail keyload from file if already loaded

Systemd mount generator: don't fail keyload from file if already loaded

Previously the generated keyload units for encryption roots with
keylocation=file://* didn't contain the code to detect if the key
was already loaded and would be marked failed in such situations.

Move the code to check whether the key is already loaded
from keylocation=prompt handling to general key loading code.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: InsanePrawn <insane.prawny@gmail.com>
Closes #10103