selftests/bpf: Improve test coverage for kfunc call

selftests/bpf: Improve test coverage for kfunc call

On powerpc, immediate load instructions are sign extended. In case
of unsigned types, arguments should be explicitly zero-extended by
the caller. For kfunc call, this needs to be handled in the JIT code.
In bpf_kfunc_call_test4(), that tests for sign-extension of signed
argument types in kfunc calls, add some additional failure checks.
And add bpf_kfunc_call_test5() to test zero-extension of unsigned
argument types in kfunc calls.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260312080113.843408-1-hbathini@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>