IB/uverbs: Fix reference counting on error paths

If an operation fails after incrementing an object's reference count,
then it should decrement the reference count on the error path.

Signed-off-by: Jack Morgenstein <jackm@mellanox.co.il>
Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>