bcachefs: Only run 'increase_depth' for keys from btree node csan

bch2_btree_increase_depth() was originally for disaster recovery, to get
some data back from the journal when a btree root was bad.

We don't need it for that purpose anymore; on bad btree root we'll
launch btree node scan and reconstruct all the interior nodes.

If there's a key in the journal for a depth that doesn't exists, and
it's not from check_topology/btree node scan, we should just ignore it.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>