bcachefs: Fix journal replay with unreadable btree roots

When a btree root is unreadable, we still might be able to get some data
back by replaying what's in the journal. Previously though, we got
confused when journal replay would attempt to replay a key for a level
that didn't exist.

This adds bch2_btree_increase_depth(), so that journal replay can handle
this.

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