ipv6: Partial checksum only UDP packets

ip6_append_data is used by other protocols and some of them can't
be partially checksummed.  Only partially checksum UDP protocol.

Fixes: 32dce968 (ipv6: Allow for partial checksums on non-ufo packets)
Reported-by: Sabrina Dubroca <sd@queasysnail.net>
Tested-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Vladislav Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>