af_key: Fix af_key.c compiler warning

net/key/af_key.c: In function ‘pfkey_spddelete’:
net/key/af_key.c:2359: warning: ‘pol_ctx’ may be used uninitialized in 
this function

When CONFIG_SECURITY_NETWORK_XFRM isn't set, 
security_xfrm_policy_alloc() is an inline that doesn't set pol_ctx, so 
this seemed like the easiest fix short of using *uninitialized_var(pol_ctx).

Signed-off-by: Brian Haley <brian.haley@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>