ipvs: correct usage/allocation of seqadj ext in ipvs

The IPVS FTP helper ip_vs_ftp could trigger an OOPS in nf_ct_seqadj_set,
after commit 41d73ec0 (netfilter: nf_conntrack: make sequence number
adjustments usuable without NAT).

This is because, the seqadj ext is now allocated dynamically, and the
IPVS code didn't handle this situation.  Fix this in the IPVS nfct
code by invoking the alloc function nfct_seqadj_ext_add().

Fixes: 41d73ec0 (netfilter: nf_conntrack: make sequence number adjustments usuable without NAT)
Suggested-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>