bcachefs: Simplify btree key cache fill path

Don't allocate the new bkey_cached until after we've done the btree
lookup; this means we can kill bkey_cached.valid.

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

Author: Kent Overstreet

fs/bcachefs/btree_iter.c

@@ -1800,13 +1800,12 @@ struct bkey_s_c bch2_btree_path_peek_slot(struct btree_path *path, struct bkey *
 			goto hole;
 	} else {
 		struct bkey_cached *ck = (void *) path->l[0].b;
-
-		EBUG_ON(ck &&
-			(path->btree_id != ck->key.btree_id ||
-			 !bkey_eq(path->pos, ck->key.pos)));
-		if (!ck || !ck->valid)
+		if (!ck)
 			return bkey_s_c_null;
 
+		EBUG_ON(path->btree_id != ck->key.btree_id ||
+			!bkey_eq(path->pos, ck->key.pos));
+
 		*u = ck->k->k;
 		k = bkey_i_to_s_c(ck->k);
 	}