mm: Introduce VM_SHADOW_STACK for shadow stack memory

New hardware extensions implement support for shadow stack memory, such
as x86 Control-flow Enforcement Technology (CET). Add a new VM flag to
identify these areas, for example, to be used to properly indicate shadow
stack PTEs to the hardware.

Shadow stack VMA creation will be tightly controlled and limited to
anonymous memory to make the implementation simpler and since that is all
that is required. The solution will rely on pte_mkwrite() to create the
shadow stack PTEs, so it will not be required for vm_get_page_prot() to
learn how to create shadow stack memory. For this reason document that
VM_SHADOW_STACK should not be mixed with VM_SHARED.

Co-developed-by: Rick Edgecombe <[email protected]>
Signed-off-by: Yu-cheng Yu <[email protected]>
Signed-off-by: Rick Edgecombe <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Reviewed-by: Borislav Petkov (AMD) <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Reviewed-by: Kirill A. Shutemov <[email protected]>
Reviewed-by: Mark Brown <[email protected]>
Acked-by: Mike Rapoport (IBM) <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Tested-by: Mark Brown <[email protected]>
Tested-by: Pengfei Xu <[email protected]>
Tested-by: John Allen <[email protected]>
Tested-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/all/20230613001108.3040476-15-rick.p.edgecombe%40intel.com

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