PKCS#7: Add OIDs for sha224, sha284 and sha512 hash algos and use them

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>