vfs: Make fs_parse() handle fs_param_is_fd-type params better

Make fs_parse() handle fs_param_is_fd-type parameters that are passed a
string by converting it to an integer (in addition to handling direct fd
specification).

Also range check the integer.

[fix from  Yin Fengwei folded]
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/fs_parser.c

@@ -204,9 +204,23 @@ int fs_parse(struct fs_context *fc,
 		goto okay;
 
 	case fs_param_is_fd: {
-		if (param->type != fs_value_is_file)
+		switch (param->type) {
+		case fs_value_is_string:
+			if (!result->has_value)
+				goto bad_value;
+
+			ret = kstrtouint(param->string, 0, &result->uint_32);
+			break;
+		case fs_value_is_file:
+			result->uint_32 = param->dirfd;
+			ret = 0;
+		default:
 			goto bad_value;
-		goto okay;
+		}
+
+		if (result->uint_32 > INT_MAX)
+			goto bad_value;
+		goto maybe_okay;
 	}
 
 	case fs_param_is_blockdev: