crypto: skcipher - fold skcipher_walk_skcipher() into skcipher_walk_virt()

Fold skcipher_walk_skcipher() into skcipher_walk_virt() which is its
only remaining caller.  No change in behavior.

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