crypto: ahash - make hash walk functions from ahash.c public

Make the hash walk functions
  crypto_hash_walk_done()
  crypto_hash_walk_first()
  crypto_hash_walk_last()
public again.
These functions had been removed from the header file
include/crypto/internal/hash.h with commit 7fa48173
("crypto: ahash - make hash walk functions private to ahash.c")
as there was no crypto algorithm code using them.

With the upcoming crypto implementation for s390 phmac
these functions will be exploited and thus need to be
public within the kernel again.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Acked-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>