crypto: des/3des_ede - add new helpers to verify keys

The recently added helper routine to perform key strength validation
of triple DES keys is slightly inadequate, since it comes in two versions,
neither of which are highly useful for anything other than skciphers (and
many drivers still use the older blkcipher interfaces).

So let's add a new helper and, considering that this is a helper function
that is only intended to be used by crypto code itself, put it in a new
des.h header under crypto/internal.

While at it, implement a similar helper for single DES, so that we can
start replacing the pattern of calling des_ekey() into a temp buffer
that occurs in many drivers in drivers/crypto.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>