net: move the hsize check to the else block in skb_segment

After commit 89319d38 ("net: Add frag_list support to skb_segment"),
it goes to process frag_list when !hsize in skb_segment(). However, when
using skb frag_list, sg normally should not be set. In this case, hsize
will be set with len right before !hsize check, then it won't go to
frag_list processing code.

So the right thing to do is move the hsize check to the else block, so
that it won't affect the !hsize check for frag_list processing.

v1->v2:
  - change to do "hsize <= 0" check instead of "!hsize", and also move
    "hsize < 0" into else block, to save some cycles, as Alex suggested.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>