IPoIB: Consolidate private neighbour data handling

Consolidate IPoIB's private neighbour data handling into
ipoib_neigh_alloc() and ipoib_neigh_free().  This will make it easier
to keep track of the neighbour structures that IPoIB is handling, and
is a nice cleanup of the code:

add/remove: 2/1 grow/shrink: 1/8 up/down: 100/-178 (-78)
function                                     old     new   delta
ipoib_neigh_alloc                              -      61     +61
ipoib_neigh_free                               -      36     +36
ipoib_mcast_join_finish                     1288    1291      +3
path_rec_completion                          575     573      -2
ipoib_mcast_join_task                        664     660      -4
ipoib_neigh_destructor                       101      92      -9
ipoib_neigh_setup_dev                         14       3     -11
ipoib_neigh_setup                             17       -     -17
path_free                                    238     215     -23
ipoib_mcast_free                             329     306     -23
ipoib_mcast_send                             718     684     -34
neigh_add_path                               705     650     -55

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>