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 <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Magnus Karlsson <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/test_xsk.sh

···
 
 if [ $failures -eq 0 ]; then
         echo "All tests successful!"
+else
+	exit 1
 fi