bcachefs: Don't persistently run scan_for_btree_nodes

bch2_btree_lost_data() gets called on btree node read error, but the
error might be transient.

btree_node_scan is expensive, and there's no need to run it persistently
(marking it in the superblock as required to run) - check_topology
will run it if required, via bch2_get_scanned_nodes().

Running it non-persistently is fine, to avoid check_topology having to
rewind recovery to run it.

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