cryptsetup: Do not fallback to PLAIN mapping if LUKS data device set fails.

cryptsetup: Do not fallback to PLAIN mapping if LUKS data device set fails.

If crypt_load() for LUKS succeeds, we know that it is a LUKS device.
Failure of data device setting should fail in this case; remapping
as a PLAIN device late could mean data corruption.

(If a user wants to map PLAIN device over a device with LUKS header,
it should be said explicitly with "plain" argument type.)

Also, if there is no explicit PLAIN type requested and crypt device
is already initialized (crypt_data_type() is set), do not run
the initialization again.