crypto: hash - Add real ahash walk interface

Although the existing hash walk interface has already been used
by a number of ahash crypto drivers, it turns out that none of
them were really asynchronous.  They were all essentially polling
for completion.

That's why nobody has noticed until now that the walk interface
couldn't work with a real asynchronous driver since the memory
is mapped using kmap_atomic.

As we now have a use-case for a real ahash implementation on x86,
this patch creates a minimal ahash walk interface.  Basically it
just calls kmap instead of kmap_atomic and does away with the
crypto_yield call.  Real ahash crypto drivers don't need to yield
since by definition they won't be hogging the CPU.

Change-Id: Id938ef6397399b779db20be9747841e9885d85b6
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>