Allow authentication data to be stored in an asymmetric key in the 4th
element of the key payload and provide a way for it to be destroyed.

For the public key subtype, this will be a public_key_signature struct.
Signed-off-by: NDavid Howells <dhowells@redhat.com>

Merge the type-specific data with the payload data into one four-word chunk
as it seems pointless to keep them separate.

Use user_key_payload() for accessing the payloads of overloaded
user-defined keys.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
cc: linux-cifs@vger.kernel.org
cc: ecryptfs@vger.kernel.org
cc: linux-ext4@vger.kernel.org
cc: linux-f2fs-devel@lists.sourceforge.net
cc: linux-nfs@vger.kernel.org
cc: ceph-devel@vger.kernel.org
cc: linux-ima-devel@lists.sourceforge.net

Create a key type that can be used to represent an asymmetric key type for use
in appropriate cryptographic operations, such as encryption, decryption,
signature generation and signature verification.

The key type is "asymmetric" and can provide access to a variety of
cryptographic algorithms.

Possibly, this would be better as "public_key" - but that has the disadvantage
that "public key" is an overloaded term.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>