netfilter: nf_conntrack_extend: avoid unnecessary "ct->ext" dereferences

As Linus points out, "ct->ext" and "new" are always equal, avoid unnecessary
dereferences and use "new" directly.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>