crypto: ahash - make hash walk functions private to ahash.c

Due to the removal of the Niagara2 SPU driver, crypto_hash_walk_first(),
crypto_hash_walk_done(), crypto_hash_walk_last(), and struct
crypto_hash_walk are now only used in crypto/ahash.c.  Therefore, make
them all private to crypto/ahash.c.  I.e. un-export the two functions
that were exported, make the functions static, and move the struct
definition to the .c file.  As part of this, move the functions to
earlier in the file to avoid needing to add forward declarations.

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