cifs: populate empty hostnames for extra channels

Currently, the secondary channels of a multichannel session
also get hostname populated based on the info in primary channel.
However, this will end up with a wrong resolution of hostname to
IP address during reconnect.

This change fixes this by not populating hostname info for all
secondary channels.

Fixes: 5112d80c ("cifs: populate server_hostname for extra channels")
Cc: stable@vger.kernel.org
Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>