pid: fix the do_each_pid_task() macro

Impact: macro side-effects fix

This patch adds parenthesis around 'pid' in the do_each_pid_task
macro to allow callers to pass in more complex parameters.

e.g.  do_each_pid_task(*pid, type, task)

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>