[IPV6]: Fix reversed local_df test in ip6_fragment

I managed to reverse the local_df test when forward-porting this
patch so it actually makes things worse by never fragmenting at
all.

Thanks to David Stevens for testing and reporting this bug.

Bill Fink pointed out that the local_df setting is also the wrong
way around.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>