MINOR: raw_sock: Report the number of bytes emitted using the splicing

In the continuity of the commit 7cf0e4517 ("MINOR: raw_sock: report global
traffic statistics"), we are now able to report the global number of bytes
emitted using the splicing. It can be retrieved in "show info" output on the
CLI.

Note this counter is always declared, regardless the splicing support. This
eases the integration with monitoring tools plugged on the CLI.
4 files changed