[PATCH] pacct: add pacct_struct to fix some pacct bugs.

The pacct facility need an i/o operation when an accounting record is
generated.  There is a possibility to wake OOM killer up.  If OOM killer is
activated, it kills some processes to make them release process memory
regions.

But acct_process() is called in the killed processes context before calling
exit_mm(), so those processes cannot release own memory.  In the results, any
processes stop in this point and it finally cause a system stall.

kernel/fork.c

@@ -874,6 +874,7 @@ static inline int copy_signal(unsigned long clone_flags, struct task_struct * ts
 		tsk->it_prof_expires =
 			secs_to_cputime(sig->rlim[RLIMIT_CPU].rlim_cur);
 	}
+	acct_init_pacct(&sig->pacct);
 
 	return 0;
 }