While converting ecryptfs over to skcipher I found that it needs
to pick a default key size if one isn't given.  Rather than having
it poke into the guts of the algorithm to get max_keysize, let's
provide a helper that is meant to give a sane default (just in
case we ever get an algorithm that has no maximum key size).

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>