nfc: virtual_ncidev: Add variable to check if ndev is running

syzbot reported an memory leak that happens when an skb is add to
send_buff after virtual nci closed.
This patch adds a variable to track if the ndev is running before
handling new skb in send function.

Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
Reported-by:  <syzbot+6eb09d75211863f15e3e@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/lkml/00000000000075472b06007df4fb@google.com


Reviewed-by: Bongsu Jeon
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>