rbd: warning, ‘devno’ may be used uninitialized in this function

rbd: warning, ‘devno’ may be used uninitialized in this function

The following warning appears during make. Fixed in both unmap_image() functions in krbd.cc

--
krbd.cc: In function ‘int krbd_unmap_by_spec(krbd_ctx*, const char*, const char*, const char*, const char*)’:
krbd.cc:608:65: warning: ‘devno’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   return do_unmap(ctx->udev, devno, build_unmap_buf(id, options));
                                                                 ^
krbd.cc:591:9: note: ‘devno’ was declared here
   dev_t devno;
--

Signed-off-by: Jos Collin <jcollin@redhat.com>