[IPV4/6]: Fix UFO error propagation

When ufo_append_data fails err is uninitialized, but returned back.
Strangely gcc doesn't notice it.

Coverity #901 and #902

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc net/ipv4/ip_output.c
Simple merge

diff --cc net/ipv6/ip6_output.c
Simple merge