Crypto/chcr: Calculate src and dst sg lengths separately for dma map

This patch calculates src and dst sg lengths separately for
dma mapping in case of aead operation.

This fixes a panic which occurs due to the accessing of a zero
length sg.
Panic:
[  138.173225] kernel BUG at drivers/iommu/intel-iommu.c:1184!

Signed-off-by: Ayush Sawal <ayush.sawal@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>