nvmetcli: set up the target only after the network is configured

nvmetcli: set up the target only after the network is configured

network.target only indicates that the network stack is up, but it
doesn't mean that the network devices have been configured.

Replace it with network-online.target, this fixes the following
error when systemd restores the target configuration during boot:

[   19.613251] nvmet_tcp: failed to bind port socket -99
[FAILED] Failed to start Restore NVMe kernel target configuration.

Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>