Use the generic crypto_authenc_extractkeys helper instead of custom
parsing code that is slightly broken.  Also fix a number of memory
leaks by moving memory allocation from setkey to init_tfm (setkey
can be called multiple times over the life of a tfm).

Finally accept all hash key lengths by running the digest over
extra-long keys.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>