rbd: check for overflow in rbd_get_num_segments()

The return type of rbd_get_num_segments() is int, but the values it
operates on are u64.  Although it's not likely, there's no guarantee
the result won't exceed what can be respresented in an int.  The
function is already designed to return -ERANGE on error, so just add
this possible overflow as another reason to return that.

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