crypto: skcipher - Fix skcipher_dequeue_givcrypt NULL test

As struct skcipher_givcrypt_request includes struct crypto_request
at a non-zero offset, testing for NULL after converting the pointer
returned by crypto_dequeue_request does not work.  This can result
in IPsec crashes when the queue is depleted.

This patch fixes it by doing the pointer conversion only when the
return value is non-NULL.  In particular, we create a new function
__crypto_dequeue_request that does the pointer conversion.

Reported-by: Brad Bosch <bradbosch@comcast.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>