ipvs: fix active FTP

- Do not create expectation when forwarding the PORT
  command to avoid blocking the connection. The problem is that
  nf_conntrack_ftp.c:help() tries to create the same expectation later in
  POST_ROUTING and drops the packet with "dropping packet" message after
  failure in nf_ct_expect_related.

- Change ip_vs_update_conntrack to alter the conntrack
  for related connections from real server. If we do not alter the reply in
  this direction the next packet from client sent to vport 20 comes as NEW
  connection. We alter it but may be some collision happens for both
  conntracks and the second conntrack gets destroyed immediately. The
  connection stucks too.

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>