x86: tls prevent_tail_call

Fix a kernel bug (vmware boot problem) reported by Tomasz Grobelny,
which occurs with certain .config variants and gccs.

The x86 TLS cleanup in commit efd1ca5
made the sys_set_thread_area and sys_get_thread_area functions ripe for
tail call optimization.  If the compiler chooses to use it for them, it
can clobber the user trap frame because these are asmlinkage functions.

Reported-by: Tomasz Grobelny <tomasz@grobelny.oswiecenia.net>
Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/tls.c

@@ -91,7 +91,9 @@ int do_set_thread_area(struct task_struct *p, int idx,
 
 asmlinkage int sys_set_thread_area(struct user_desc __user *u_info)
 {
-	return do_set_thread_area(current, -1, u_info, 1);
+	int ret = do_set_thread_area(current, -1, u_info, 1);
+	prevent_tail_call(ret);
+	return ret;
 }
 
 
@@ -139,7 +141,9 @@ int do_get_thread_area(struct task_struct *p, int idx,
 
 asmlinkage int sys_get_thread_area(struct user_desc __user *u_info)
 {
-	return do_get_thread_area(current, -1, u_info);
+	int ret = do_get_thread_area(current, -1, u_info);
+	prevent_tail_call(ret);
+	return ret;
 }
 
 int regset_tls_active(struct task_struct *target,