selftests/bpf: Fix out-of-bounds array access bugs reported by ASAN

selftests/bpf: Fix out-of-bounds array access bugs reported by ASAN

- kmem_cache_iter: remove unnecessary debug output
- lwt_seg6local: change the type of foobar to char[]
  - the sizeof(foobar) returned the pointer size and not a string
    length as intended
- verifier_log: increase prog_name buffer size in verif_log_subtest()
  - compiler has a conservative estimate of fixed_log_sz value, making
    ASAN complain on snprint() call

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260223191118.655185-1-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>