mtd: mtdpart: fix add_mtd_partitions error path

If we fail to allocate a partition structure in the middle of the partition
creation process, the already allocated partitions are never removed, which
means they are still present in the partition list and their resources are
never freed.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: stable@vger.kernel.org
Signed-off-by: Brian Norris <computersforpeace@gmail.com>