dm verity: fix verity_parse_opt_args parsing

Commit df326e7a ("dm verity: allow optional args to alter primary
args handling") introduced a bug where verity_parse_opt_args() wouldn't
properly shift past an optional argument's additional params (by
ignoring them).

Fix this by avoiding returning with error if an unknown argument is
encountered when @only_modifier_opts=true is passed to
verity_parse_opt_args().

In practice this regressed the cryptsetup testsuite's FEC testing
because unknown optional arguments were encountered, wherey
short-circuiting ever testing FEC mode. With this fix all of the
cryptsetup testsuite's verity FEC tests pass.

Fixes: df326e7a ("dm verity: allow optional args to alter primary args handling")
Reported-by: Milan Broz <gmazyland@gmail.com&gt;>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-verity-target.c

@@ -1052,7 +1052,7 @@ static int verity_parse_opt_args(struct dm_arg_set *as, struct dm_verity *v,
 				 struct dm_verity_sig_opts *verify_args,
 				 bool only_modifier_opts)
 {
-	int r;
+	int r = 0;
 	unsigned argc;
 	struct dm_target *ti = v->ti;
 	const char *arg_name;
@@ -1122,8 +1122,18 @@ static int verity_parse_opt_args(struct dm_arg_set *as, struct dm_verity *v,
 			if (r)
 				return r;
 			continue;
+
+		} else if (only_modifier_opts) {
+			/*
+			 * Ignore unrecognized opt, could easily be an extra
+			 * argument to an option whose parsing was skipped.
+			 * Normal parsing (@only_modifier_opts=false) will
+			 * properly parse all options (and their extra args).
+			 */
+			continue;
 		}
 
+		DMERR("Unrecognized verity feature request: %s", arg_name);
 		ti->error = "Unrecognized verity feature request";
 		return -EINVAL;
 	} while (argc && !r);