[IA64] find thread for user rbs address

I encountered one problem when running ptrace test case the situation
is this: traced process's syscall parameter needs to be accessed, but
for sys_clone system call with clone_flag (CLONE_VFORK | CLONE_VM |
SIGCHLD) parameter.  This syscall's parameter accessing result is wrong.

The reason is that vforked child process mm point is the same, but
tgid is different. Without this patch find_thread_for_addr will return
vforked process if vforked process is also stopped, but not the thread
which calls vfork syscall.
Signed-off-by: NTony Luck <tony.luck@intel.com>

arch/ia64/kernel/ptrace.c

@@ -607,7 +607,7 @@ find_thread_for_addr (struct task_struct *child, unsigned long addr)
 	 */
  	list_for_each_safe(this, next, &current->children) {
 		p = list_entry(this, struct task_struct, sibling);
-		if (p->mm != mm)
+		if (p->tgid != child->tgid)
 			continue;
 		if (thread_matches(p, addr)) {
 			child = p;