Revert "Store block last offset instead of block size"

Revert "Store block last offset instead of block size"

This reverts commit d0ec586c02c441b352efb1ec763270e84335f5ca.

It turns out that using max offset instead of size was a bad idea.
Among other things that broke, tree search may return a zero-sized
insert block at offset 0 for any position, because its size is -1,
which overflows to UOFF_MAX, so it appears to be a block that covers
all possible file positions...

Of course, this change breaks handling of (UOFF_MAX+1)-sized blocks,
but that will be much easier to fix.

Signed-off-by: Petr Tesarik <petr@tesarici.cz>