nvme-tcp: use __dev_get_by_name instead dev_get_by_name for OPT_HOST_IFACE

dev_get_by_name() finds network device by name but it also increases the
reference count.

If a nvme-tcp queue is present and the network device driver is removed
before nvme_tcp, we will face the following continuous log:

  "kernel:unregister_netdevice: waiting for <eth> to become free. Usage count = 2"

And rmmod further halts. Similar case arises during reboot/shutdown
with nvme-tcp queue present and both never completes.

To fix this, use __dev_get_by_name() which finds network device by
name without increasing any reference counter.

Fixes: 3ede8f72

 ("nvme-tcp: allow selecting the network interface for connections")
Signed-off-by: Omkar Kulkarni <okulkarni@marvell.com>
Signed-off-by: Shai Malin <smalin@marvell.com>
Signed-off-by: Prabhakar Kushwaha <pkushwaha@marvell.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
[hch: remove the ->ndev member entirely]
Signed-off-by: Christoph Hellwig <hch@lst.de>