libceph: fix null dereference when unregistering linger requests
We should only clear r_osd if we are neither registered as a linger or a
regular request. We may unregister as a linger while still registered as
a regular request (e.g., in reset_osd). Incorrectly clearing r_osd there
leads to a null pointer dereference in __send_request.
Also simplify the parallel check in __unregister_request() where we just
removed r_osd_item and know it's empty.