RDMA/uverbs: Factor out the add/get pattern into a helper

The next patch needs another copy of this, provide a simple helper to
reduce the coding. uapi_add_get_elm() returns an existing entry or adds a
new one.

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