SUNRPC: Hoist KDF into struct gss_krb5_enctype

Each Kerberos enctype can have a different KDF. Refactor the key
derivation path to support different KDFs for the enctypes
introduced in subsequent patches.

In particular, expose the key derivation function in struct
gss_krb5_enctype instead of the enctype's preferred random-to-key
function. The latter is usually the identity function and is only
ever called during key derivation, so have each KDF call it
directly.

A couple of extra clean-ups:
- Deduplicate the set_cdata() helper
- Have ->derive_key return negative errnos, in accordance with usual
  kernel coding conventions

This patch is a little bigger than I'd like, but these are all
mechanical changes and they are all to the same areas of code. No
behavior change is intended.

Tested-by: Scott Mayhew <[email protected]>
Reviewed-by: Simo Sorce <[email protected]>
Signed-off-by: Chuck Lever <[email protected]>

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