selftests/bpf: Replace strncpy() with strscpy()

selftests/bpf: Replace strncpy() with strscpy()

strncpy() does not guarantee NULL-termination and is considered
deprecated [1]. Replace strncpy() calls with strscpy().

[1] https://docs.kernel.org/process/deprecated.html#strncpy-on-nul-terminated-strings

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260223190736.649171-4-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>