netfilter: allow hooks to pass error code back up the stack

SELinux would like to pass certain fatal errors back up the stack.  This patch
implements the generic netfilter support for this functionality.

Based-on-patch-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>