x86: Don't let pgprot_modify() change the page encryption bit

When SEV or SME is enabled and active, vm_get_page_prot() typically
returns with the encryption bit set. This means that users of
pgprot_modify(, vm_get_page_prot()) (mprotect_fixup(), do_mmap()) end up
with a value of vma->vm_pg_prot that is not consistent with the intended
protection of the PTEs.

This is also important for fault handlers that rely on the VMA
vm_page_prot to set the page protection. Fix this by not allowing
pgprot_modify() to change the encryption bit, similar to how it's done
for PAT bits.

Signed-off-by: Thomas Hellstrom <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Reviewed-by: Dave Hansen <[email protected]>
Acked-by: Tom Lendacky <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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