RDMA/uverbs: Do not discard the IB_EVENT_DEVICE_FATAL event

The commit below moved all of the destruction to the disassociate step and
cleaned up the event channel during destroy_uobj.

However, when ib_uverbs_free_hw_resources() pushes IB_EVENT_DEVICE_FATAL
and then immediately goes to destroy all uobjects this causes
ib_uverbs_free_event_queue() to discard the queued event if userspace
hasn't already read() it.

Unlike all other event queues async FD needs to defer the
ib_uverbs_free_event_queue() until FD release. This still unregisters the
handler from the IB device during disassociation.

Fixes: 3e032c0e ("RDMA/core: Make ib_uverbs_async_event_file into a uobject")
Link: https://lore.kernel.org/r/20200507063348.98713-2-leon@kernel.org


Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>