lguest: prepare to make SWITCHER_ADDR a variable.

We currently use the whole top PGD entry for the switcher, but that's
hitting the fixmap in some configurations (mainly, large NR_CPUS).
Introduce a variable, currently set to the constant.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

arch/x86/include/asm/lguest.h

@@ -23,6 +23,8 @@
 #else
 #define SWITCHER_ADDR 0xFFC00000
 #endif
+/* Where we map the Switcher, in both Host and Guest. */
+extern unsigned long switcher_addr;
 
 /* Found in switcher.S */
 extern unsigned long default_idt_entries[];