Mask CPU id to 16 bits when encoding block req_id

Make the high-16-bits CPU field explicit at the point of use so the
encoding matches its doc comment and no longer relies on silent shift
overflow to bound the CPU id. Behaviorally equivalent for realistic CPU
counts; the mask just documents the field width in the code itself.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015Y9in1s3yxA9ZWLKmfQyUa

Author: claude

src/tracer/prober/prober.c

@@ -2898,7 +2898,7 @@ TRACEPOINT_PROBE(block, block_rq_issue) {
   if (gen) {
     u64 seq = *gen;
     *gen = seq + 1;
-    ictx.req_id = ((u64)bpf_get_smp_processor_id() << 48) | (seq & 0xFFFFFFFFFFFFULL);
+    ictx.req_id = ((u64)(bpf_get_smp_processor_id() & 0xFFFF) << 48) | (seq & 0xFFFFFFFFFFFFULL);
   }
 
   block_start_times.update(&key, &ictx);