efi/x86: Preserve %ebx correctly in efi_set_virtual_address_map()

Commit:

  59f2a619 ("efi: Add 'runtime' pointer to struct efi")

modified the assembler routine called by efi_set_virtual_address_map(),
to grab the 'runtime' EFI service pointer while running with paging
disabled (which is tricky to do in C code)

After the change, register %ebx is not restored correctly, resulting
in all kinds of weird behavior, so fix that.
Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200304133515.15035-1-ardb@kernel.org
Link: https://lore.kernel.org/r/20200308080859.21568-22-ardb@kernel.org

arch/x86/platform/efi/efi_stub_32.S

@@ -54,7 +54,7 @@ SYM_FUNC_START(efi_call_svam)
 	orl	$0x80000000, %edx
 	movl	%edx, %cr0
 
-	pop	%ebx
+	movl	16(%esp), %ebx
 	leave
 	ret
 SYM_FUNC_END(efi_call_svam)