selftests: net: run reuseport in an isolated netns

selftests: net: run reuseport in an isolated netns

The reuseport_* tests (bpf, bpf_cpu, bpf_numa, dualstack) currently use
a fixed port range. This can cause intermittent test failures when the
ports are already in use by other services:

  failed to bind receive socket: Address already in use

To avoid conflicts, run these tests in separate network namespaces using
unshare. Each test now has its own isolated network stack, preventing
port collisions with the host services.

Signed-off-by: Aleksei Oladko <aleksey.oladko@virtuozzo.com>
Link: https://patch.msgid.link/20260321215908.175465-2-aleksey.oladko@virtuozzo.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>