ipv6: Remove superfluous NULL pointer check in ipv6_local_rxpmtu

The pointer to mtu_info is taken from the common buffer
of the skb, thus it can't be a NULL pointer. This patch
removes this check on mtu_info.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>