Revert "net: Remove DECnet leftovers from flow.h."

This reverts commit bf82668e which is
commit 9bc61c04 upstream.

It is removing unused code, but that changes a structure size so revert
it for now to preserve the ABI.

Bug: 161946584
Change-Id: I237cda2e5c07440d25613a1a1e30fa499751c7ac
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>