perf_counter tools: Add locking to perf top

We need to protect the active_symbols list as two threads change it:
the main thread adding entries to the head and the display thread
decaying entries from any place in the list.

Also related: take a snapshot of syme->count[0] before using it to
calculate the weight and to show the same number used in this calc when
displaying the symbol usage.
Reported-by: Mike Galbraith <efault@gmx.de>
Tested-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <20090529200307.GR4747@ghostprotocols.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>