[CRYPTO] api: Explain the comparison on larval cra_name

This patch adds a comment to explain why we compare the cra_driver_name of
the algorithm being registered against the cra_name of a larval as opposed
to the cra_driver_name of the larval.

In fact larvals have only one name, cra_name which is the name that was
requested by the user.  The test here is simply trying to find out whether
the algorithm being registered can or can not satisfy the larval.

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