crypto: kpp - provide support for KPP template instances

The upcoming support for the RFC 7919 ffdhe group parameters will be
made available in the form of templates like "ffdhe2048(dh)",
"ffdhe3072(dh)" and so on. Template instantiations thereof would wrap the
inner "dh" kpp_alg and also provide kpp_alg services to the outside again.
Furthermore, it might be perhaps be desirable to provide KDF templates in
the future, which would similarly wrap an inner kpp_alg and present
themselves to the outside as another kpp_alg, transforming the shared
secret on its way out.

Introduce the bits needed for supporting KPP template instances. Everything
related to inner kpp_alg spawns potentially being held by such template
instances will be deferred to a subsequent patch in order to facilitate
review.

Define struct struct kpp_instance in close analogy to the already existing
skcipher_instance, shash_instance and alike, but wrapping a struct kpp_alg.
Implement the new kpp_register_instance() template instance registration
primitive. Provide some helper functions for
- going back and forth between a generic struct crypto_instance and the new
  struct kpp_instance,
- obtaining the instantiating kpp_instance from a crypto_kpp transform and
- for accessing a given kpp_instance's implementation specific context
  data.

Annotate everything with proper kernel-doc comments, even though
include/crypto/internal/kpp.h is not considered for the generated docs.

Signed-off-by: Nicolai Stange <nstange@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>