net: ipv6: fix returned variable type in ip6_skb_dst_mtu

The patch fixing the returned value of ip6_skb_dst_mtu (int -> unsigned
int) was rebased between its initial review and the version applied. In
the meantime fade5641 was applied, which added a new variable (int)
used as the returned value. This lead to a mismatch between the function
prototype and the variable used as the return value.

Fixes: 40fc3054

 ("net: ipv6: fix return value of ip6_skb_dst_mtu")
Cc: Vadim Fedorenko <vfedorenko@novek.ru>
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>