ipvs: avoid oops for passive FTP

Fix Passive FTP problem in ip_vs_ftp:

- Do not oops in nf_nat_set_seq_adjust (adjust_tcp_sequence) when
  iptable_nat module is not loaded

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>