RDMA/core: Make ib_uverbs_async_event_file into a uobject

This makes async events aligned with completion events as both are full
uobjects of FD type and use the same uobject lifecycle.

A bunch of duplicate code is consolidated and the general flow between the
two FDs is now very similar.

Link: https://lore.kernel.org/r/1578504126-9400-14-git-send-email-yishaih@mellanox.com


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