selftests/bpf: Skip spin lock failure test on s390x

Instead of adding the whole test to DENYLIST.s390x, which also has
success test cases that should be run, just skip over failure test
cases in case the JIT does not support kfuncs.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20221118185938.2139616-3-memxor@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>