ceph: don't crash when passed bad mount options

This only happened when parse_extra_token was not passed
to ceph_parse_option() (hence, only happened in rbd).

Signed-off-by: Yehuda Sadeh <[e-mail]>

net/ceph/ceph_common.c

···
 			continue;
 		err = -EINVAL;
 		token = match_token((char *)c, opt_tokens, argstr);
-		if (token < 0) {
+		if (token < 0 && parse_extra_token) {
 			/* extra? */
 			err = parse_extra_token((char *)c, private);
 			if (err < 0) {