When scatterwalk is built as a module digest.c was broken because it
requires the crypto_km_types structure which is in scatterwalk.  This
patch removes the crypto_km_types structure by encoding the logic into
crypto_kmap_type directly.

In fact, this even saves a few bytes of code (not to mention the data
structure itself) on i386 which is about the only place where it's
needed.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>