smb3: fix empty netname context on secondary channels

Some servers do not allow null netname contexts, which would cause
multichannel to revert to single channel when mounting to some
servers (e.g. Azure xSMB).

Fixes: 4c14d704 ("cifs: populate empty hostnames for extra channels")
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>