Don't log expected error on DFS referral request

STATUS_FS_DRIVER_REQUIRED is expected when DFS is not turned
on on the server.  Do not log it on DFS referral response.
It clutters the dmesg log unnecessarily at mount time.

Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Reviewed-by: Ronnie sahlberg <lsahlber@redhat.com>