netconsole: do not release spin_lock when calling __netpoll_cleanup

With the previous patch applied, __netpoll_cleanup() is non-block now,
so we don't need to release the spin_lock before calling it.

Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>