IB/uverbs: Add an ib_uobject getter to ioctl() infrastructure

Previously, the user had to dig inside the attribute to get the uobject.
Add a helper function that correctly extract it (and do the required
checks) for him/her.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>