xfrm: get global statistics from the offloaded device

Iterate over all SAs in order to fill global IPsec statistics.

Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>