mm: multi-gen LRU: fix crash during cgroup migration

In the Linux kernel, the following vulnerability has been resolved:

mm: multi-gen LRU: fix crash during cgroup migration

lru_gen_migrate_mm() assumes lru_gen_add_mm() runs prior to itself. This

isn't true for the following scenario:

CPU 1 CPU 2

clone()

cgroup_can_fork()

cgroup_procs_write()

cgroup_post_fork()

task_lock()

lru_gen_migrate_mm()

task_unlock()

task_lock()

lru_gen_add_mm()

task_unlock()

And when the above happens, kernel crashes because of linked list

corruption (mm_struct->lru_gen.list).