Add ib_ucontext to the uverbs_attr_bundle sent down the iocl and cmd flows
as soon as the flow has ib_uobject.

In addition, remove rdma_get_ucontext helper function that is only used by
ib_umem_get.
Signed-off-by: Shamir Rabinovitch <shamir.rabinovitch@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>