doc: networking: convert offload files into RST and update references

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: Otto Sabart <[e-mail]>
Acked-by: David S. Miller <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

include/linux/skbuff.h

···
 /* Local Checksum Offload.
  * Compute outer checksum based on the assumption that the
  * inner checksum will be offloaded later.
- * See Documentation/networking/checksum-offloads.txt for
+ * See Documentation/networking/checksum-offloads.rst for
  * explanation of how this works.
  * Fill in outer checksum adjustment (e.g. with sum of outer
  * pseudo-header) before calling.