crypto: skcipher - add crypto_skcipher_min_keysize()

Add a helper function crypto_skcipher_min_keysize() to mirror
crypto_skcipher_max_keysize().

This will be used by the self-tests.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>