libceph: fix a osd request memory leak

If an invalid layout is provided to ceph_osdc_new_request(), its
call to calc_layout() might return an error.  At that point in the
function we've already allocated an osd request structure, so we
need to free it (drop a reference) in the event such an error
occurs.

The only other value calc_layout() will return is 0, so make that
explicit in the successful case.

This resolves:
    http://tracker.ceph.com/issues/4240



Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>