[IPV4]: Change ip_defrag to return an integer

Now that ip_frag always returns the packet given to it on input, we can
change it to return an integer indicating error instead.  This patch does
that and updates all its callers accordingly.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>