IB/uverbs: Tidy uverbs_uobject_add

Maintaining the uobjects list is mandatory, hoist it into the common
rdma_alloc_commit_uobject() function and inline it as there is now
only one caller.

Signed-off-by: Leon Romanovsky <leon@kernel.org>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>