selftests/bpf: Rename libarena struct bitmap to struct arena_bitmap

When building bpf selftest with latest bpf-next, I got the following failure:

  In file included from /home/yhs/work/bpf-next/tools/testing/selftests/bpf/libarena/selftests/test_parallel_bitmap.bpf.c:8:
  /home/yhs/work/bpf-next/tools/testing/selftests/bpf/libarena/include/libarena/bitmap.h:11:8: error: redefinition of
        'bitmap'
     11 | struct bitmap {
        |        ^
  /home/yhs/work/bpf-next/tools/testing/selftests/bpf/tools/include/vmlinux.h:51320:8: note: previous definition is here
   51320 | struct bitmap {
         |        ^

The vmlinux.h struct bitmap comes from drivers/md/md-bitmap.c:
  struct bitmap {
	struct bitmap_counts { ... }
	...
  }

To fix the issue, I renamed libarena struct bitmap to arena_bitmap to avoid the conflict.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/bpf/20260707220136.910374-1-yonghong.song@linux.dev
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>