bcachefs: Fix hidden btree errors when reading roots

We silence btree errors in btree_node_scan, since it's probing and
errors are expected: add a fake pass so that btree_node_scan is no
longer recovery pass 0, and we don't think we're in btree node scan when
reading btree roots.

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