crypto: skcipher - document skcipher_walk_done() and rename some vars

skcipher_walk_done() has an unusual calling convention, and some of its
local variables have unclear names.  Document it and rename variables to
make it a bit clearer what is going on.  No change in behavior.

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