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 <ottosabart@seberm.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

include/linux/skbuff.h

@@ -4296,7 +4296,7 @@ static inline bool skb_head_is_locked(const struct sk_buff *skb)
 /* 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.