perf: Remove superfluous return values from perf_callchain_*()

Fixes these build warnings introduced by the callchain
rework:

 arch/x86/kernel/cpu/perf_event.c: In function ‘perf_callchain_kernel’:
 arch/x86/kernel/cpu/perf_event.c:1646: warning: ‘return’ with a value, in function returning void
 arch/x86/kernel/cpu/perf_event.c: In function ‘perf_callchain_user’:
 arch/x86/kernel/cpu/perf_event.c:1699: warning: ‘return’ with a value, in function returning void
 arch/x86/kernel/cpu/perf_event.c: At top level:
 arch/x86/kernel/cpu/perf_event.c:1607: warning: ‘perf_callchain_entry_nmi’ defined but not used

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/cpu/perf_event.c

@@ -1610,7 +1610,7 @@ perf_callchain_kernel(struct perf_callchain_entry *entry, struct pt_regs *regs)
 {
 	if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
 		/* TODO: We don't support guest os callchain now */
-		return NULL;
+		return;
 	}
 
 	perf_callchain_store(entry, regs->ip);
@@ -1663,7 +1663,7 @@ perf_callchain_user(struct perf_callchain_entry *entry, struct pt_regs *regs)
 
 	if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
 		/* TODO: We don't support guest os callchain now */
-		return NULL;
+		return;
 	}
 
 	fp = (void __user *)regs->bp;