ceph: avoid null deref in osd request error path

If we interrupt an osd request, we call __cancel_request, but it wasn't
verifying that req->r_osd was non-NULL before dereferencing it.  This could
cause a crash if osds were flapping and we aborted a request on said osd.

Reported-by: Henry C Chang <henry_c_chang@tcloudcomputing.com>
Signed-off-by: Sage Weil <sage@newdream.net>