selftests/bpf: add verification for BPF_PROG_QUERY attr size boundaries

Add a new selftest to verify that the BPF syscall (specifically
BPF_PROG_QUERY) correctly handles different user-declared attribute sizes.

Specifically, verify that:
- For cgroup queries, a query with a size that covers 'prog_cnt' but is
  smaller than 'revision' (OLD_QUERY_SIZE) succeeds, but does not write
  to 'revision' (verifying backward compatibility).
- A query with full size (FULL_QUERY_SIZE) succeeds and writes both
  'prog_cnt' and 'revision'.

Fixes: 120933984460 ("bpf: Implement mprog API on top of existing cgroup progs")
Cc: Maciej Żenczykowski <maze@google.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Yuyang Huang <yuyanghuang@google.com>
Link: https://lore.kernel.org/r/20260531075600.4058207-3-yuyanghuang@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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