RDMA/core: Add UVERBS_METHOD_ASYNC_EVENT_ALLOC

Allow the async FD to be allocated separately from the context.

This is necessary to introduce the ioctl to create a context, as an ioctl
should only ever create a single uobject at a time.

If multiple async FDs are created then the first one is used to deliver
affiliated events from any ib_uevent_object, with all subsequent ones will
receive only unaffiliated events.

Link: https://lore.kernel.org/r/1578506740-22188-3-git-send-email-yishaih@mellanox.com
Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>