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: Richard Weinberger <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/um/os-Linux/process.c

···
 		    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)