dnsdist-1.9.x: Backport of 13914 - Fix XSK-enabled check when reconnecting a backend

[rgacogne]

Short description

Backport of #13914 to 1.9.x:

The check was reverted, meaning that we did not properly remove an address:port from the XSK map when reconnecting a backend, and actually tried to remove an entry from a non-existing map when XSK is compiled in but not enabled, triggering a:

Error getting the XSK destination addresses map path '/sys/fs/bpf/dnsdist/xsk-destinations-v4'

message.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
• checked that this code was merged to master

[coveralls]

Pull Request Test Coverage Report for Build 8340748746

Details

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 55.51%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/dnsdistdist/dnsdist-backend.cc	0	1	0.0%

Totals
Change from base Build 8262456785:	0.02%
Covered Lines:	47445
Relevant Lines:	64958

---

💛 - Coveralls