tty: fix memory leak in vc_deallocate

stable inclusion
from linux-4.19.191
commit 3ce3062bcf01de94815605f1b6fd916cb84ff08b

--------------------------------

commit 211b4d42b70f1c1660feaa968dac0efc2a96ac4d upstream.

syzbot reported memory leak in tty/vt.
The problem was in VT_DISALLOCATE ioctl cmd.
After allocating unimap with PIO_UNIMAP it wasn't
freed via VT_DISALLOCATE, but vc_cons[currcons].d was
zeroed.

Reported-by:  <syzbot+bcc922b19ccc64240b42@syzkaller.appspotmail.com>
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210327214443.21548-1-paskripkin@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>