selftests/bpf: test_xsk: Don't exit immediately when xsk_attach fails

selftests/bpf: test_xsk: Don't exit immediately when xsk_attach fails

xsk_reattach_xdp calls exit_with_error() on failures. This exits the
program immediately. It prevents the following tests from being run and
isn't compliant with the CI.

Add a return value to the functions handling XDP attachments to handle
errors 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-v7-9-39fe486593a3@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>