xfrm: Remove unnecessary NULL check in xfrm_lookup_with_ifid()

This NULL check is unnecessary and can be removed.  It confuses
Smatch static analysis tool because it makes Smatch think that
xfrm_lookup_with_ifid() can return a mix of NULL pointers and errors so
it creates a lot of false positives.  Remove it.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Michal Kubiak <michal.kubiak@intel.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>