fork.c: cleanup for copy_sighand()

Check CLONE_SIGHAND only is enough, because combination of CLONE_THREAD and
CLONE_SIGHAND is already done in copy_process().

Impact: cleanup, no functionality changed
Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/fork.c

@@ -758,7 +758,7 @@ static int copy_sighand(unsigned long clone_flags, struct task_struct *tsk)
 {
 	struct sighand_struct *sig;
 
-	if (clone_flags & (CLONE_SIGHAND | CLONE_THREAD)) {
+	if (clone_flags & CLONE_SIGHAND) {
 		atomic_inc(&current->sighand->count);
 		return 0;
 	}