Call kthread_exit() rather than kproc_exit() for a premature kthread exit.

Kernel threads (and processes) are supposed to call kthread_exit() (or
kproc_exit()) to terminate.  However, the kernel includes a fallback in
fork_exit() to force a kthread exit if a kernel thread's "main" routine
returns.  This fallback was added back when the kernel only had processes
and was not updated to call kthread_exit() instead of kproc_exit() when
threads were added to the kernel.

This mistake was particular exciting when the errant thread belonged to
proc0.  Due to the missing P_KTHREAD flag the fallback did not kick in
and instead tried to return to userland via whatever garbage was in the
trapframe.  With P_KTHREAD set it tried to terminate proc0 resulting in
other amusements.

PR:		204999
MFC after:	1 week

sys/kern/kern_fork.c

@@ -1040,7 +1040,7 @@ fork_exit(void (*callout)(void *, struct trapframe *), void *arg,
 	if (p->p_flag & P_KTHREAD) {
 		printf("Kernel thread \"%s\" (pid %d) exited prematurely.\n",
 		    td->td_name, p->p_pid);
-		kproc_exit(0);
+		kthread_exit();
 	}
 	mtx_assert(&Giant, MA_NOTOWNED);