fscrypt: clean up fscrypt_add_test_dummy_key()

Now that fscrypt_add_test_dummy_key() is only called by
setup_file_encryption_key() and not by the individual filesystems,
un-export it.  Also change its prototype to take the
fscrypt_key_specifier directly, as the caller already has it.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20230208062107.199831-6-ebiggers@kernel.org