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>