RDMA/odp: remove ib_ucontext from ib_umem

At this point the ucontext is only being stored to access the ib_device,
so just store the ib_device directly instead. This is more natural and
logical as the umem has nothing to do with the ucontext.

Link: https://lore.kernel.org/r/20190806231548.25242-8-jgg@ziepe.ca


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