selftests/bpf: Upon failures, exit with code 1 in test_xsk.sh

Currently, even if some subtests fails, the end result will still yield
"ok 1 selftests: bpf: test_xsk.sh". Fix it by exiting with 1 if there are
any failures.

Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20250828-selftests-bpf-test_xsk_ret-v1-1-e6656c01f397@suse.com