f2fs: reject test_dummy_encryption when !CONFIG_FS_ENCRYPTION

There is no good reason to allow this mount option when the kernel isn't
configured with encryption support.  Since this option is only for
testing, we can just fix this; we don't really need to worry about
breaking anyone who might be counting on this option being ignored.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>