IB/core: Enable getting an object type from a given uobject

Enable getting an object type from a given uobject, the type is saved
upon tree merging and is returned as part of some helper function.

Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>