tracing: protect reader of cmdline output

Impact: fix to one cause of incorrect comm outputs in trace

The spinlock only protected the creation of a comm <=> pid pair.
But it was possible that a reader could look up a pid, and get the
wrong comm because it had no locking.

This also required changing trace_find_cmdline to copy the comm cache
and not just send back a pointer to it.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>

kernel/trace/trace.h

@@ -547,7 +547,7 @@ struct tracer_switch_ops {
 };
 #endif /* CONFIG_CONTEXT_SWITCH_TRACER */
 
-extern char *trace_find_cmdline(int pid);
+extern void trace_find_cmdline(int pid, char comm[]);
 
 #ifdef CONFIG_DYNAMIC_FTRACE
 extern unsigned long ftrace_update_tot_cnt;