x86: Fix build regression with CONFIG_KEXEC_JUMP enabled

BugLink: https://bugs.launchpad.net/bugs/2102118

[ Upstream commit aeb68937614f4aeceaaa762bd7f0212ce842b797 ]

Build 6.13-rc12 for x86_64 with gcc 14.2.1 fails with the error:

  ld: vmlinux.o: in function `virtual_mapped':
  linux/arch/x86/kernel/relocate_kernel_64.S:249:(.text+0x5915b): undefined reference to `saved_context_gdt_desc'

when CONFIG_KEXEC_JUMP is enabled.

This was introduced by commit 07fa619f2a40 ("x86/kexec: Restore GDT on
return from ::preserve_context kexec") which introduced a use of
saved_context_gdt_desc without a declaration for it.

Fix that by including asm/asm-offsets.h where saved_context_gdt_desc
is defined (indirectly in include/generated/asm-offsets.h which
asm/asm-offsets.h includes).

Fixes: 07fa619f2a40 ("x86/kexec: Restore GDT on return from ::preserve_context kexec")
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: David Woodhouse <dwmw@amazon.co.uk>
Closes: https://lore.kernel.org/oe-kbuild-all/202411270006.ZyyzpYf8-lkp@intel.com/
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

arch/x86/kernel/relocate_kernel_64.S

@@ -11,6 +11,7 @@
 #include <asm/pgtable_types.h>
 #include <asm/nospec-branch.h>
 #include <asm/unwind_hints.h>
+#include <asm/asm-offsets.h>
 
 /*
  * Must be relocatable PIC code callable as a C function, in particular