geneve: add dsfield helper function

Add a helper function to compute the tos/dsfield. In this way, we can
factor out some duplicate code. Also, the helper will be called from
more places in the next commit.

Suggested-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>