net: Abstract dst->neighbour accesses behind helpers.

drivers/infiniband/core/addr.c

drivers/infiniband/hw/cxgb3/iwch_cm.c

drivers/infiniband/hw/cxgb4/cm.c

drivers/infiniband/hw/nes/nes_cm.c

drivers/infiniband/ulp/ipoib/ipoib_main.c

drivers/infiniband/ulp/ipoib/ipoib_multicast.c

drivers/net/cxgb3/cxgb3_offload.c

drivers/s390/net/qeth_l3_main.c

drivers/scsi/cxgbi/cxgb3i/cxgb3i.c

drivers/scsi/cxgbi/cxgb4i/cxgb4i.c

drivers/scsi/cxgbi/libcxgbi.c

include/net/dst.h

net/atm/clip.c

net/bridge/br_netfilter.c

net/core/dst.c

net/core/neighbour.c

net/decnet/dn_neigh.c

net/decnet/dn_route.c

net/ipv4/ip_gre.c

net/ipv4/ip_output.c