smb: client: improve DFS mount check

Some servers may return error codes from REQ_GET_DFS_REFERRAL requests
that are unexpected by the client, so to make it easier, assume
non-DFS mounts when the client can't get the initial DFS referral of
@ctx->UNC in dfs_mount_share().

Signed-off-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>