um: fix crash while os_dump_core()

os_dump_core() emits SIGTERM to terminate all UML processes.  Kernel
threads have to exit on SIGTERM instead of calling last_ditch_exit().
Multiple calls to last_ditch_exit() can cause a crash.
Signed-off-by: NRichard Weinberger <richard@nod.at>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/um/os-Linux/process.c

@@ -253,6 +253,7 @@ void init_new_thread_signals(void)
 		    SA_ONSTACK | SA_RESTART, SIGUSR1, SIGIO, SIGWINCH, SIGALRM,
 		    SIGVTALRM, -1);
 	signal(SIGWINCH, SIG_IGN);
+	signal(SIGTERM, SIG_DFL);
 }
 
 int run_kernel_thread(int (*fn)(void *), void *arg, jmp_buf **jmp_ptr)