xfrm: don't check offload_handle for nonzero

The offload_handle should be an opaque data cookie for the driver
to use, much like the data cookie for a timer or alarm callback.
Thus, the XFRM stack should not be checking for non-zero, because
the driver might use that to store an array reference, which could
be zero, or some other zero but meaningful value.

We can remove the checks for non-zero because there are plenty
other attributes also being checked to see if there is an offload
in place for the SA in question.

Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>