xfrm: enforce validity of offload input flags

struct xfrm_user_offload has flags variable that received user input,
but kernel didn't check if valid bits were provided. It caused a situation
where not sanitized input was forwarded directly to the drivers.

For example, XFRM_OFFLOAD_IPV6 define that was exposed, was used by
strongswan, but not implemented in the kernel at all.

As a solution, check and sanitize input flags to forward
XFRM_OFFLOAD_INBOUND to the drivers.

Fixes: d77e38e6 ("xfrm: Add an IPsec hardware offloading API")
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>