sit: Clean up DF code by copying from IPIP

This patch rearranges the SIT DF bit handling using the new IPIP DF
code.  The only externally visible effect should be the case where
PMTU is enabled and the MTU is exactly 1280 bytes.  In this case the
previous code would send packets out with DF off while the new code
would set the DF bit.  This is inline with RFC 4213.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Thanks,
Signed-off-by: David S. Miller <davem@davemloft.net>