rbd: work around -Wuninitialized warning

gcc -O3 warns about a dummy variable that is passed
down into rbd_img_fill_nodata without being initialized:

drivers/block/rbd.c: In function 'rbd_img_fill_nodata':
drivers/block/rbd.c:2573:13: error: 'dummy' is used uninitialized in this function [-Werror=uninitialized]
  fctx->iter = *fctx->pos;

Since this is a dummy, I assume the warning is harmless, but
it's better to initialize it anyway and avoid the warning.

Fixes: mmtom ("init/Kconfig: enable -O3 for all arches")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>