When we find an unreachable inode, we now reattach it in the oldest
version that needs to be reattached (thus avoiding redundant work
reattaching every single version), and we now fix up inode -> dirent
backpointers in newer versions as needed - or white out the reattaching
dirent in newer versions, if the newer version isn't supposed to be
reattached.

This results in the second verify fsck now passing cleanly after
repairing on a user-provided filesystem image with thousands of
different snapshots.
Reported-by: Christopher Snowhill <chris@kode54.net>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>