libceph: fix linger request requeueing

Fix the request transition from linger -> normal request.  The key is to
preserve r_osd and requeue on the same OSD.  Reregister as a normal request,
add the request to the proper queues, then unregister the linger.  Fix the
unregister helper to avoid clearing r_osd (and also simplify the parallel
check in __unregister_request()).

Reported-by: Henry Chang <henry.cy.chang@gmail.com>
Signed-off-by: Sage Weil <sage@newdream.net>