librbd: flush on diff_iterate

The diff_iterate() tests fail when caching is enabled because recent writes
aren't visible to listsnaps.  Flush from diff_iterate to ensure that they
are.  Someday, maybe, we might make diff_iterate() inspect the cache
contents to make this more efficient, but for now that is not necessary.

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