exec: Remove FOLL_FORCE for stack setup

It does not appear that FOLL_FORCE should be needed for setting up the
stack pages. They are allocated using the nascent brpm->vma, which was
newly created with VM_STACK_FLAGS, which an arch can override, but they
all appear to include VM_WRITE | VM_MAYWRITE. Remove FOLL_FORCE.

Cc: Eric Biederman <ebiederm@xmission.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
Link: https://lore.kernel.org/lkml/202211171439.CDE720EAD@keescook/


Signed-off-by: Kees Cook <keescook@chromium.org>