x86/sev: Run RMPADJUST on SVSM calling area page to test VMPL

x86/sev: Run RMPADJUST on SVSM calling area page to test VMPL

Determining the VMPL at which the kernel runs involves performing a RMPADJUST
operation on an arbitrary page of memory, and observing whether it succeeds.

The use of boot_ghcb_page in the core kernel in this case is completely
arbitrary, but results in the need to provide a PIC alias for it. So use
boot_svsm_ca_page instead, which already needs this alias for other reasons.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/20250828102202.1849035-28-ardb+git@google.com