x86/paravirt: finish change from lazy cpu to context switch start/end

Impact: fix lazy context switch API

Pass the previous and next tasks into the context switch start
end calls, so that the called functions can properly access the
task state (esp in end_context_switch, in which the next task
is not yet completely current).

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

include/asm-generic/pgtable.h

@@ -291,7 +291,7 @@ static inline void ptep_modify_prot_commit(struct mm_struct *mm,
  * definition.
  */
 #ifndef __HAVE_ARCH_START_CONTEXT_SWITCH
-#define arch_start_context_switch()	do {} while (0)
+#define arch_start_context_switch(prev)	do {} while (0)
 #endif
 
 #ifndef __HAVE_PFNMAP_TRACKING