[NETFILTER]: nf_nat: fix MASQUERADE crash on device down

Check the return value of nfct_nat() in device_cmp(), we might very well
have non NAT conntrack entries as well (Netfilter bugzilla #528).

Signed-off-by: Martin Josefsson <gandalf@wlug.westbo.se>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>