perf_counter tools: Consolidate dso methods to load kernel symbols

Now one has just to use dso__load_kernel() optionally passing a vmlinux
filename.

Will make things easier for perf top that will want to pass a callback
to filter some symbols.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>