CVE-2023-52659: x86/mm: Ensure input to pfn_to_kaddr() is treated as a 64-bit type

In the Linux kernel, the following vulnerability has been resolved:

x86/mm: Ensure input to pfntokaddr() is treated as a 64-bit type

On 64-bit platforms, the pfntokaddr() macro requires that the input value is 64 bits in order to ensure that valid address bits don't get lost when shifting that input by PAGESHIFT to calculate the physical address to provide a virtual address for.

One such example is in pvalidatepages() (used by SEV-SNP guests), where the GFN in the struct used for page-state change requests is a 40-bit bit-field, so attempts to pass this GFN field directly into pfntokaddr() ends up causing guest crashes when dealing with addresses above the 1TB range due to the above.

Fix this issue with SEV-SNP guests, as well as any similar cases that might cause issues in current/future code, by using an inline function, instead of a macro, so that the input is implicitly cast to the expected 64-bit input type prior to performing the shift operation.

While it might be argued that the issue is on the caller side, other archs/macros have taken similar approaches to deal with instances like this, such as ARM explicitly casting the input to physaddrt:

e48866647b48 ("ARM: 8396/1: use physaddrt in pfntokaddr()")

A C inline function is even better though.

[ mingo: Refined the changelog some more & added alwaysinline. ]