[MTD] fix use after free in register_mtd_blktrans

Reported-by: Dan Carpenter <error27@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>