bcachefs: Improve a verbose log message

We should be using bch2_err_str() where applicable.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/super.c

@@ -1920,7 +1920,8 @@ struct bch_fs *bch2_fs_open(char * const *devices, unsigned nr_devices,
 	kfree(sb);
 	printbuf_exit(&errbuf);
 	module_put(THIS_MODULE);
-	pr_verbose_init(opts, "ret %i", PTR_ERR_OR_ZERO(c));
+	pr_verbose_init(opts, "ret %s (%i)", bch2_err_str(PTR_ERR_OR_ZERO(c)),
+			PTR_ERR_OR_ZERO(c));
 	return c;
 err_print:
 	pr_err("bch_fs_open err opening %s: %s",