mtd: virt_concat: fix use-after-free in mtd_virt_concat_destroy_joins()

mtd: virt_concat: fix use-after-free in mtd_virt_concat_destroy_joins()

mtd_concat_destroy() frees item->concat so calling
mtd_virt_concat_put_mtd_devices(item->concat) leads to a use after free.

Fix this by moving mtd_virt_concat_put_mtd_devices() before
mtd_concat_destroy()

Fixes: 43db6366fc2d ("mtd: Add driver for concatenating devices")
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>