RDMA/mlx5: Fix async events cleanup flows

RDMA/mlx5: Fix async events cleanup flows

As in the prior patch, the devx code is not fully cleaning up its
event_lists before finishing driver_destroy allowing a later read to
trigger user after free conditions.

Re-arrange things so that the event_list is always empty after destroy and
ensure it remains empty until the file is closed.

Fixes: f7c8416ccea5 ("RDMA/core: Simplify destruction of FD uobjects")
Link: https://lore.kernel.org/r/20200212072635.682689-7-leon@kernel.org
Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>