Initialize the structure to 0 so that it's fields won't have random
values. For example fields like rec.traffic_class (as well as
rec.flow_label and rec.sl) is used to generate the user AH through:
  cma_iboe_join_multicast
    cma_make_mc_event
      ib_init_ah_from_mcmember

And a random traffic_class causes a random IP DSCP in RoCEv2.

Fixes: b5de0c60 ("RDMA/cma: Fix use after free race in roce multicast join")
Signed-off-by: Mark Zhang <markzhang@nvidia.com>
Link: https://lore.kernel.org/r/20230927090511.603595-1-markzhang@nvidia.comSigned-off-by: Leon Romanovsky <leon@kernel.org>