selftests/bpf: Add DENYLIST.asan

selftests/bpf: Add DENYLIST.asan

Add a denylist file for tests that should be skipped when built with
userspace ASAN:

    $ make ... SAN_CFLAGS="-fsanitize=address -fno-omit-frame-pointer"

Skip the following tests:

- *arena*: userspace ASAN does not understand BPF arena maps and gets
  confused particularly when map_extra is non-zero
  - non-zero map_extra leads to mmap with MAP_FIXED, and ASAN treats
    this as an unknown memory region
- task_local_data: ASAN complains about "incorrect" aligned_alloc()
  usage, but it's intentional in the test
- uprobe_multi_test: very slow with ASAN enabled

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260223190736.649171-9-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>