When we find an extent past an inode's i_size, we need to do the
deletion in the inode's snapshot (which will emit a whiteout if
necessary); and we also need to note that we now have an a key at that
position and snapshot, so that we don't go into an infinite loop.

Also, switch to walking inodes in reverse older, oldest snapshot to
newest, so that we emit the fewest whiteouts possible.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>