perf session: Validate nr fields against event size on both swap and common paths

perf session: Validate nr fields against event size on both swap and common paths

Several event types use an nr field to control iteration over
variable-length arrays.  The swap handlers byte-swap and loop using
these fields without bounds checks, and the native processing path
trusts them as well.

Add bounds checks on both paths for:

- PERF_RECORD_THREAD_MAP: validate nr against payload, return -1
  on the swap path.  On the native path, reject with -EINVAL.

- PERF_RECORD_NAMESPACES: clamp nr on the swap path (safe because
  each entry is indexed by type; missing entries just won't be
  resolved).  Skip the event on the native path.

- PERF_RECORD_CPU_MAP: clamp nr for CPUS and MASK sub-types on
  the swap path.  Add bounds checks for mask64 which previously
  had no nr validation.  Skip the event on the native path.

- PERF_RECORD_STAT_CONFIG: clamp nr on the swap path (safe because
  each config entry is self-describing via its tag).  Skip the
  event on the native path.

The swap path (cross-endian, writable MAP_PRIVATE mapping) can
safely clamp by writing back to the event.  The native path
(read-only MAP_SHARED mapping) must skip instead of clamping
because writing to the mmap'd event would segfault.

Also fix stat_config swap range: change size += 1 to
size += sizeof(event->stat_config.nr) for clarity.  The old +1
happened to work because mem_bswap_64 processes 8-byte chunks,
but the intent is to include the 8-byte nr field in the swap
range.

Changes in v2:
- Document that PERF_RECORD_NAMESPACES max_nr includes trailing
  sample_id space when sample_id_all is present — harmless on the
  swap path because both per-element bswap_64 and swap_sample_id_all()
  perform the same u64 byte swap (Reported-by: sashiko-bot@kernel.org)

Reported-by: sashiko-bot@kernel.org # Running on a local machine
Reviewed-by: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Assisted-by: Claude:claude-opus-4.6-1m
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>