cifs: dns resolution is needed only for primary channel

When calling cifs_reconnect, before the connection to the
server is reestablished, the code today does a DNS resolution and
updates server->dstaddr.

However, this is not necessary for secondary channels. Secondary
channels use the interface list returned by the server to decide
which address to connect to. And that happens after tcon is reconnected
and server interfaces are requested.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Cc: stable@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>