Fix panic with RACCT that could occur in low memory (or out of swap)

situations, due to fork1() calling racct_proc_exit() without calling
racct_proc_fork() first.

Submitted by:	Mateusz Guzik <mjguzik at gmail dot com> (earlier version)
Reviewed by:	Mateusz Guzik <mjguzik at gmail dot com>

sys/kern/kern_fork.c

@@ -939,8 +939,8 @@ fork1(struct thread *td, int flags, int pages, struct proc **procp,
 #ifdef MAC
 	mac_proc_destroy(newproc);
 #endif
-fail1:
 	racct_proc_exit(newproc);
+fail1:
 	if (vm2 != NULL)
 		vmspace_free(vm2);
 	uma_zfree(proc_zone, newproc);

sys/kern/kern_racct.c

@@ -573,6 +573,9 @@ racct_proc_fork(struct proc *parent, struct proc *child)
 	PROC_UNLOCK(child);
 	PROC_UNLOCK(parent);
 
+	if (error != 0)
+		racct_proc_exit(child);
+
 	return (error);
 }