librbd: change diff_iterate interface to be more C-friendly

librbd: change diff_iterate interface to be more C-friendly

Use int instead of bool for the callback, and make it represent
whether the data exists, rather than the opposite, since callers
are likely to test for whether it's data instead of whether its zeroes.

Change the return value to 0, since an int64_t will wrap around
for large reads, and there's no value in reporting the length
read when it will always be the length requested clipped to the
size of the image.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>