ip_vti: remove the useless err_count check in vti_xmit

Unlike ipip and gre, ip_vti never uses err_count in vti4_err,
so no need to check err_count in vti_xmit, it's value always 0.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>