c/r: prctl: drop VMA flags test on PR_SET_MM_ stack data assignment

c/r: prctl: drop VMA flags test on PR_SET_MM_ stack data assignment

In commit b76437579d13 ("procfs: mark thread stack correctly in
proc/<pid>/maps") the stack allocated via clone() is marked in
/proc/<pid>/maps as [stack:%d] thus it might be out of the former
mm->start_stack/end_stack values (and even has some custom VMA flags
set).

So to be able to restore mm->start_stack/end_stack drop vma flags test,
but still require the underlying VMA to exist.

As always note this feature is under CONFIG_CHECKPOINT_RESTORE and
requires CAP_SYS_RESOURCE to be granted.

Signed-off-by: Cyrill Gorcunov <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Acked-by: Kees Cook <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Serge Hallyn <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>