IB/uverbs: Refactor related objects to use their own asynchronous event FD

Refactor related objects to use their own asynchronous event FD.
The ufile event FD will be the default in case an object won't have its own
event FD.

Link: https://lore.kernel.org/r/20200519072711.257271-3-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>