ceph: fix check for invalidate_mapping_pages success

We need to know whether there was any page left behind, and not the
return value (the total number of pages invalidated).  Look at the mapping
to see if we were successful or not.

Move it all into a helper to simplify the two callers.

Signed-off-by: Yehuda Sadeh <yehuda@hq.newdream.net>
Signed-off-by: Sage Weil <sage@newdream.net>