librbd: uint64_t len for diff_iterate

Signed-off-by: Sage Weil <sage@inktank.com>
librbd: document diff_iterate in header

Signed-off-by: Sage Weil <sage@inktank.com>
librbd: diff_iterate fromsnapname after the end snap is also invalid

Signed-off-by: Sage Weil <sage@inktank.com>