MD: fix null pointer deference

The md device might not have personality (for example, ddf raid array). The
issue is introduced by 8430e7e0(md: disconnect device from personality
before trying to remove it)
Reported-by: Nkernel test robot <xiaolong.ye@intel.com>
Signed-off-by: NShaohua Li <shli@fb.com>

drivers/md/md.c

@@ -2595,8 +2595,10 @@ state_store(struct md_rdev *rdev, const char *buf, size_t len)
 		else
 			err = -EBUSY;
 	} else if (cmd_match(buf, "remove")) {
+		if (rdev->mddev->pers) {
 			clear_bit(Blocked, &rdev->flags);
 			remove_and_add_spares(rdev->mddev, rdev);
+		}
 		if (rdev->raid_disk >= 0)
 			err = -EBUSY;
 		else {