This patch renames offload files. This is necessary for Sphinx.

Also update reference to checksum-offloads.rst file.

Whole kernel code was grepped for references using:
$ grep -r "\(segmentation\|checksum\)-offloads.txt" .

There should be no other references
to {segmentation,checksum}-offloads.txt files.
Signed-off-by: NOtto Sabart <ottosabart@seberm.com>
Acked-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>