IB/uverbs: Clarify the kref'ing ordering for alloc_commit

The alloc_commit callback makes the uobj visible to other threads,
and it does so using a 'move' semantic of the uobj kref on the stack
into the public storage (eg the IDR, uobject list and file_private_data)

Once this is done another thread could start up and trigger deletion
of the kref. Fortunately cleanup_rwsem happens to prevent this from
being a bug, but that is a fantastically unclear side effect.

Re-organize things so that alloc_commit is that last thing to touch
the uobj, get rid of the sneaky implicit dependency on cleanup_rwsem,
and add a comment reminding that uobj is no longer kref'd after
alloc_commit.

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