IB/uverbs: Enable device removal when there are active user space applications

Enables the uverbs_remove_one to succeed despite the fact that there are
running IB applications working with the given ib device.  This
functionality enables a HW device to be unbind/reset despite the fact that
there are running user space applications using it.

It exposes a new IB kernel API named 'disassociate_ucontext' which lets
a driver detaching its HW resources from a given user context without
crashing/terminating the application. In case a driver implemented the
above API and registered with ib_uverb there will be no dependency between its
device to its uverbs_device. Upon calling remove_one of ib_uverbs the call
should return after disassociating the open HW resources without waiting to
clients disconnecting. In case driver didn't implement this API there will be no
change to current behaviour and uverbs_remove_one will return only when last
client has disconnected and reference count on uverbs device became 0.

In case the lower driver device was removed any application will
continue working over some zombie HCA, further calls will ended with an
immediate error.

Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Shachar Raindel <raindel@mellanox.com>
Reviewed-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>