perf symbols: Filter samples with unresolved symbol when "--symbols" option is used

perf symbols: Filter samples with unresolved symbol when "--symbols" option is used

Report/top commands support to only handle specific symbols with
"--symbols" option, but current code will keep those samples whose
symbol can't be resolved, which should actually be filtered.

If we run following commands:
$perf record -a tree
$perf report --symbols intel_idle -n
the output will be:

Without the patch:
==================
    46.27%        156     sshd  [unknown]
    26.05%         48  swapper  [kernel.kallsyms]
    17.26%         38     tree  libc-2.12.1.so
     7.69%         17     tree  tree
     2.73%          6     tree  ld-2.12.1.so

With the patch:
===============
   100.00%         48  swapper  [kernel.kallsyms]

Signed-off-by: Feng Tang <feng.tang@intel.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1347007349-3102-2-git-send-email-feng.tang@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>