Revert "net: ipv4: ip_forward: fix inverted local_df test"

This reverts commit 59d9f389df3cdf72833d5ee17c3fe959b6bdc792, which
was commit ca6c5d4ad216d5942ae544bbf02503041bd802aa upstream.  It is a
valid fix, but depends on sk_buff::local_df being set in all the right
cases, which it wasn't in 3.2.  We need to defer it unless and until
the other fixes are also backported to 3.2.y.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>