skbuff: skb_segment: s/frag/nskb_frag/

[ Upstream commit 8cb19905 ]

frag points at nskb, so name it appropriately

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>