bridge: superfluous skb->nfct check in br_nf_dev_queue_xmit

Currently bridge can silently drop ipv4 fragments.
If node have loaded nf_defrag_ipv4 module but have no nf_conntrack_ipv4,
br_nf_pre_routing defragments incoming ipv4 fragments
but nfct check in br_nf_dev_queue_xmit does not allow re-fragment combined
packet back, and therefore it is dropped in br_dev_queue_push_xmit without
incrementing of any failcounters

It seems the only way to hit the ip_fragment code in the bridge xmit
path is to have a fragment list whose reassembled fragments go over
the mtu. This only happens if nf_defrag is enabled. Thanks to
Florian Westphal for providing feedback to clarify this.

Defragmentation ipv4 is required not only in conntracks but at least in
TPROXY target and socket match, therefore #ifdef is changed from
NF_CONNTRACK_IPV4 to NF_DEFRAG_IPV4

Signed-off-by: Vasily Averin <vvs@openvz.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>