x86/kexec: Ensure preserve_context flag is set on return to kernel

The swap_pages() function will only actually *swap*, as its name implies, if
the preserve_context flag in the %r11 register is non-zero. On the way back
from a ::preserve_context kexec, ensure that the %r11 register is non-zero so
that the pages get swapped back.

Fixes: 9e5683e ("x86/kexec: Only swap pages for ::preserve_context mode")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20250109140757.2841269-3-dwmw2@infradead.org

Author: dwmw2

arch/x86/kernel/relocate_kernel_64.S

@@ -220,6 +220,7 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	movq	kexec_pa_table_page(%rip), %rax
 	movq	%rax, %cr3
 	lea	PAGE_SIZE(%r8), %rsp
+	movl	$1, %r11d	/* Ensure preserve_context flag is set */
 	call	swap_pages
 	movq	kexec_va_control_page(%rip), %rax
 	addq	$(virtual_mapped - relocate_kernel), %rax