bcachefs: Don't build aux search tree when still repairing node

bcachefs: Don't build aux search tree when still repairing node

bch2_btree_node_drop_keys_outside_node() will (re)build aux search
trees, because it's also called by topology repair.

bch2_btree_node_read_done() was calling it before validating individual
keys; invalid ones have to be dropped.

If we call drop_keys_outside_node() first, then
bch2_bset_build_aux_tree() doesn't run because the node already has an
aux search tree - which was invalidated by the repair.

Reported-by: syzbot+c5e7a66b3b23ae65d44f@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>