bcachefs: Fix restart handling in btree_node_scrub_work()

bcachefs: Fix restart handling in btree_node_scrub_work()

btree node scrub was sometimes failing to rewrite nodes with errors;
bch2_btree_node_rewrite() can return a transaction restart and we
weren't checking - the lockrestart_do() needs to wrap the entire
operation.

And there's a better helper it should've been using,
bch2_btree_node_rewrite_key(), which makes all this more convenient.

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