With the new btree node scan code, we can now recover from corrupt btree
roots - simply create a new fake root at depth 1, and then insert all
the leaves we found.

If the root wasn't corrupt but there's corruption elsewhere in the
btree, we can fill in holes as needed with the newest version of a given
node(s) from the scan; we also check if a given btree node is older than
what we found from the scan.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>