alpha: fix crash if pthread_create races with signal delivery

On alpha, a process will crash if it attempts to start a thread and a
signal is delivered at the same time. The crash can be reproduced with
this program: https://cygwin.com/ml/cygwin/2014-11/msg00473.html

The reason for the crash is this:
* we call the clone syscall
* we go to the function copy_process
* copy process calls copy_thread_tls, it is a wrapper around copy_thread
* copy_thread sets the tls pointer: childti->pcb.unique = regs->r20
* copy_thread sets regs->r20 to zero
* we go back to copy_process
* copy process checks "if (signal_pending(current))" and returns
  -ERESTARTNOINTR
* the clone syscall is restarted, but this time, regs->r20 is zero, so
  the new thread is created with zero tls pointer
* the new thread crashes in start_thread when attempting to access tls

The comment in the code says that setting the register r20 is some
compatibility with OSF/1. But OSF/1 doesn't use the CLONE_SETTLS flag, so
we don't have to zero r20 if CLONE_SETTLS is set. This patch fixes the bug
by zeroing regs->r20 only if CLONE_SETTLS is not set.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Matt Turner <mattst88@gmail.com>

arch/alpha/kernel/process.c

@@ -269,12 +269,13 @@ copy_thread(unsigned long clone_flags, unsigned long usp,
 	   application calling fork.  */
 	if (clone_flags & CLONE_SETTLS)
 		childti->pcb.unique = regs->r20;
+	else
+		regs->r20 = 0;	/* OSF/1 has some strange fork() semantics.  */
 	childti->pcb.usp = usp ?: rdusp();
 	*childregs = *regs;
 	childregs->r0 = 0;
 	childregs->r19 = 0;
 	childregs->r20 = 1;	/* OSF/1 has some strange fork() semantics.  */
-	regs->r20 = 0;
 	stack = ((struct switch_stack *) regs) - 1;
 	*childstack = *stack;
 	childstack->r26 = (unsigned long) ret_from_fork;