crypto: shash - Require all algorithms to support export/import

This patch provides a default export/import function for all
shash algorithms.  It simply copies the descriptor context as
is done by sha1_generic.

This in essence means that all existing shash algorithms now
support export/import.  This is something that will be depended
upon in implementations such as hmac.  Therefore all new shash
and ahash implementations must support export/import.

For those that cannot obtain a partial result, padlock-sha's
fallback model should be used so that a partial result is always
available.

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