crypto: skcipher - add helper for simple block cipher modes

The majority of skcipher templates (including both the existing ones and
the ones remaining to be converted from the "blkcipher" API) just wrap a
single block cipher algorithm.  This includes cbc, cfb, ctr, ecb, kw,
ofb, and pcbc.  Add a helper function skcipher_alloc_instance_simple()
that handles allocating an skcipher instance for this common case.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>