BUG/MINOR: sample: fix backend direction flags consecutive to last fix

Commit 7a06ffb85 ("BUG/MEDIUM: sample: Cumulate frontend and backend
sample validity flags") introduced a typo confusing the request and
the response direction when checking for validity of a rule applied
to a backend. This was reported by Coverity in issue #1417.

This needs to be backported where the patch above is backported.
1 file changed