libceph: avoid unregistering osd request when not registered

There is a race between two __unregister_request() callers: the
reply path and the ceph_osdc_wait_request().  If we get a reply
*and* the timeout expires at roughly the same time, both callers
will try to unregister the request, and the second one will do bad
things.

Simply check if the request is still already unregistered; if so,
return immediately and do nothing.

Fixes http://tracker.newdream.net/issues/2420



Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Alex Elder <elder@inktank.com>