memblock: use kfree() to release kmalloced memblock regions

memblock.{reserved,memory}.regions may be allocated using kmalloc() in
memblock_double_array(). Use kfree() to release these kmalloced regions
indicated by memblock_{reserved,memory}_in_slab.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Fixes: 3010f876 ("mm: discard memblock data later")
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>