perf top: Don't look for kernel idle symbols in all DSOs

perf top: Don't look for kernel idle symbols in all DSOs

The 'top' tool initially supported only kernel symbols, when making it
support userspace symbols we forgot to make the symbol filter first
check that the DSO is the kernel one. Fix it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
 c: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-54haztkeigmbump5sexxnzhv@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>