Add OIDs for sha224, sha284 and sha512 hash algos and use them to select
the hashing algorithm.  Without this, something like the following error
might get written to dmesg:

[   31.829322] PKCS7: Unknown OID: [32] 2.16.840.1.101.3.4.2.3
[   31.829328] PKCS7: Unknown OID: [180] 2.16.840.1.101.3.4.2.3
[   31.829330] Unsupported digest algo: 55

Where the 55 on the third line is OID__NR indicating an unknown OID.
Reported-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-By: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: James Morris <james.l.morris@oracle.com>