[NETFILTER]: Clear GSO bits for TCP reset packet

The TCP reset packet is copied from the original.  This
includes all the GSO bits which do not apply to the new
packet.  So we should clear those bits.

Spotted by Patrick McHardy.

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