efi: provide empty efi_enter_virtual_mode implementation

efi: provide empty efi_enter_virtual_mode implementation

When CONFIG_EFI is not enabled, we might get an undefined reference to
efi_enter_virtual_mode() error, if this efi_enabled() call isn't inlined
into start_kernel().  This happens in particular, if start_kernel() is
annodated with __no_sanitize_address.

Reported-by: kernel test robot <[email protected]>
Signed-off-by: Andrey Konovalov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: Ard Biesheuvel <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Elena Petrova <[email protected]>
Cc: Marco Elver <[email protected]>
Cc: Vincenzo Frascino <[email protected]>
Cc: Walter Wu <[email protected]>
Link: http://lkml.kernel.org/r/6514652d3a32d3ed33d6eb5c91d0af63bf0d1a0c.1596544734.git.andreyknvl@google.com
Signed-off-by: Linus Torvalds <[email protected]>