binder: use group leader instead of open thread

The binder allocator assumes that the thread that
called binder_open will never die for the lifetime of
that proc. That thread is normally the group_leader,
however it may not be. Use the group_leader instead
of current.

Signed-off-by: Todd Kjos <tkjos@google.com>
Cc: stable <stable@vger.kernel.org> # 4.4+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>