crypto: skcipher - Don't use __crypto_dequeue_request()

Use skcipher_givcrypt_cast(crypto_dequeue_request(queue)) instead, which
does the same thing in much cleaner way. The skcipher_givcrypt_cast()
actually uses container_of() instead of messing around with offsetof()
too.

Signed-off-by: Marek Vasut <marex@denx.de>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>