ipv6: remove printk

ipv6_find_hdr() prints a non-rate limited error message
when it cannot find an ipv6 header at a specific offset.
This could be used as a DoS, so just remove it.

Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>