fscrypt: factor out fscrypt_policy_to_key_spec()

Factor out a function that builds the fscrypt_key_specifier for an
fscrypt_policy.  Before this was only needed when finding the key for a
file, but now it will also be needed for test_dummy_encryption support.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20220501050857.538984-4-ebiggers@kernel.org