selftests/bpf: Systematically add SO_REUSEADDR in start_server_addr

selftests/bpf: Systematically add SO_REUSEADDR in start_server_addr

Some tests have to stop/start a server multiple time with the same
listening address. Doing so without SO_REUSADDR leads to failures due to
the socket still being in TIME_WAIT right after the first instance
stop/before the second instance start. Instead of letting each test
manually set SO_REUSEADDR on their servers, it can be done automatically
by start_server_addr for all tests (and without any major downside).

Enforce SO_REUSEADDR in start_server_addr for all tests.

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20251105-start-server-soreuseaddr-v1-1-1bbd9c1f8d65@bootlin.com