perf symbols: Ignore stripped vmlinux and fallback to kallsyms

perf symbols: Ignore stripped vmlinux and fallback to kallsyms

If a vmlinux is stripped, perf will use it and ignore kallsyms. We
end up with useless profiles where everything maps to a few
runtime symbols:

    63.39%       swapper  [kernel.kallsyms]   [k] hcall_real_table
     4.90%      beam.smp  [kernel.kallsyms]   [k] hcall_real_table
     4.44%      beam.smp  [kernel.kallsyms]   [k] __sched_text_start
     3.72%      beam.smp  [kernel.kallsyms]   [k] __run_at_kexec

Detect this case and fallback to using kallsyms. This fixes the issue:

    62.81%       swapper  [kernel.kallsyms]   [k] snooze_loop
     4.44%      beam.smp  [kernel.kallsyms]   [k] __schedule
     0.91%      beam.smp  [kernel.kallsyms]   [k] _switch
     0.73%      beam.smp  [kernel.kallsyms]   [k] put_prev_entity

Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20140909085929.4a5a81f0@kryten
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>