ima: add regular file data hash signature version 3 support

ima: add regular file data hash signature version 3 support

Instead of directly verifying the signature of a file data hash,
signature v3 verifies the signature of the ima_file_id structure
containing the file data hash.

To disambiguate the signature usage, the ima_file_id structure also
includes the hash algorithm and the type of data (e.g. regular file
hash or fs-verity root hash).

Tested-by: Stefan Berger <stefanb@linux.ibm.com>
Acked-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>