RDMA/ucma: Consolidate the two destroy flows

ucma_close() is open coding the tail end of ucma_destroy_id(), consolidate
this duplicated code into a function.

Link: https://lore.kernel.org/r/20200818120526.702120-4-leon@kernel.org


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