RDMA/uverbs: Initialize uverbs_attr_bundle ucontext in ib_uverbs_get_context

ib_uverbs_get_context does not have a uobject so it does not call the
rdma_lookup_get_uobject which is used to set up the uverbs_attr_bundle
ucontext. For ib_uverbs_get_context we need to set up this manually before
we send the uverbs_attr_bundle down to the driver layer.

This completes the change that was done in commit 70f06b26

 ("IB:
ucontext should be set properly for all cmd & ioctl paths")

Signed-off-by: Shamir Rabinovitch <shamir.rabinovitch@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>