selftests/bpf: test_xsk: Add return value to init_iface()

selftests/bpf: test_xsk: Add return value to init_iface()

init_iface() doesn't have any return value while it can fail. In case of
failure it calls exit_on_error() which exits the application
immediately. This prevents the following tests from being run and isn't
compliant with the CI

Add a return value to init_iface() so errors can be handled more
smoothly.

Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Link: https://lore.kernel.org/r/20251031-xsk-v7-8-39fe486593a3@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>