PAM: do not fail to mount if the key's already loaded

If we're expecting a working home directory on login, it would be
rather frustrating to not have it mounted just because it e.g. failed to
unmount once on logout.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Felix Dörre <felix@dogcraft.de>
Signed-off-by: Val Packett <val@packett.cool>
Closes #14834