dma_map_single() is an heavy operation which doesn't need to
be done at each request as the key doesn't change.

Instead of DMA mapping the key at every request, this patch maps it
once in setkey()
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>