exec: avoid propagating PF_NO_SETAFFINITY into userspace child

Userspace process doesn't want the PF_NO_SETAFFINITY, but its parent may be
a kernel worker thread which has PF_NO_SETAFFINITY set, and this worker thread
can do kernel_thread() to create the child.
Clearing this flag in usersapce child to enable its migrating capability.
Signed-off-by: Zhang Yi <zhang.yi20@zte.com.cn>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -1087,8 +1087,8 @@ int flush_old_exec(struct linux_binprm * bprm)
 	bprm->mm = NULL;		/* We're using it now */
 
 	set_fs(USER_DS);
-	current->flags &=
-		~(PF_RANDOMIZE | PF_FORKNOEXEC | PF_KTHREAD | PF_NOFREEZE);
+	current->flags &= ~(PF_RANDOMIZE | PF_FORKNOEXEC | PF_KTHREAD |
+					PF_NOFREEZE | PF_NO_SETAFFINITY);
 	flush_thread();
 	current->personality &= ~bprm->per_clear;