ceph: fix osd request submission race

The osd request submission path registers the request, drops and retakes
the request_mutex, then sends it to the OSD.  A racing kick_requests could
sent it during that interval, causing the same msg to be sent twice and
BUGing in the msgr.

Fix by only sending the message if it hasn't been touched by other
threads.

Signed-off-by: Sage Weil <sage@newdream.net>