selftests: netfilter: fix exit value for nft_concat_range

When the nft_concat_range test failed, it exit 1 in the code
specifically.

But when part of, or all of the test passed, it will failed the
[ ${passed} -eq 0 ] check and thus exit with 1, which is the same
exit value with failure result. Fix it by exit 0 when passed is not 0.

Fixes: 611973c1 ("selftests: netfilter: Introduce tests for sets with range concatenation")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>