RDMA/mlx5: set UMR wqe fence according to HCA cap

Cache the needed umr_fence and set the wqe ctrl segmennt
accordingly.

Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Acked-by: Leon Romanovsky <leon@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Doug Ledford <dledford@redhat.com>