[CRYPTO] api: Add crypto_attr_alg_name

This patch adds a new helper crypto_attr_alg_name which is basically the
first half of crypto_attr_alg.  That is, it returns an algorithm name
parameter as a string without looking it up.  The caller can then look it
up immediately or defer it until later.

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