netfilter: bridge: allow fragmentation of VLAN packets traversing a bridge

br_nf_dev_queue_xmit only checks for ETH_P_IP packets for fragmenting but not
VLAN packets. This results in dropping of large VLAN packets. This can be
observed when connection tracking is enabled. Connection tracking re-assembles
fragmented packets, and these have to re-fragmented when transmitting out. Also,
make sure only refragmented packets are defragmented as per suggestion from
Patrick McHardy.

Signed-off-by: Saikiran Madugula <hummerbliss@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>