selftests: pmtu: Drop prints to kernel log from pmtu_vti6_link_change_mtu

Reported-by: David Ahern <dsahern@gmail.com>
Fixes: 1fad59ea ("selftests: pmtu: Add pmtu_vti6_link_change_mtu test")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>