Since commit 41d73ec0 (netfilter: nf_conntrack: make sequence
number adjustments usuable without NAT), the sequence number extension
is dynamically allocated.

Instead of dying, give a WARN splash, in case of wrong usage of the
seqadj code, e.g. when forgetting to allocate via nfct_seqadj_ext_add().

Wrong usage have been seen in the IPVS code path.
Signed-off-by: NJesper Dangaard Brouer <brouer@redhat.com>
Acked-by: NJulian Anastasov <ja@ssi.bg>
Signed-off-by: NSimon Horman <horms@verge.net.au>