crypto: lib/sha256 - Clear W[] in sha256_update() instead of sha256_transform()

The temporary W[] array is currently zeroed out once every call to
sha256_transform(), i.e. once every 64 bytes of input data. Moving it to
sha256_update() instead so that it is cleared only once per update can
save about 2-3% of the total time taken to compute the digest, with a
reasonable memset() implementation, and considerably more (~20%) with a
bad one (eg the x86 purgatory currently uses a memset() coded in C).

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>