net: fix GSO for SG-enabled devices

The commit dbd50f23

 ("net: move the hsize check to the else
block in skb_segment") introduced a data corruption for devices
supporting scatter-gather.

The problem boils down to signed/unsigned comparison given
unexpected results: if signed 'hsize' is negative, it will be
considered greater than a positive 'len', which is unsigned.

This commit addresses resorting to the old checks order, so that
'hsize' never has a negative value when compared with 'len'.

v1 -> v2:
 - reorder hsize checks instead of explicit cast (Alex)

Bisected-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Fixes: dbd50f23

 ("net: move the hsize check to the else block in skb_segment")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/861947c2d2d087db82af93c21920ce8147d15490.1611074818.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>