[CPT] thread groups with exited leader did not migrate
The bug is simple and stupid, it is very strange nobody saw this.
When thread group leader exits, its mm/files/fs/namespace are released
but zombie process remain frozen until all the threads exit.
Restore was not able to restore such configuration.
Solution is simple: when chackpointing save not real (NULL)
mm/files/fs/namespace, but mm/files/fs/namespace of this thread group.