[Benchmark] Correctly kill vllm process in performance benchamrk (#2782)

### What this PR does / why we need it?
vLLM now names the process with VLLM prefix after
https://github.com/vllm-project/vllm/pull/21445, we should kill the
correct process name after one iteration benchmark to avoid OOM issue
### Does this PR introduce _any_ user-facing change?

### How was this patch tested?

- vLLM version: v0.10.1.1
- vLLM main:
e599e2c65e

---------

Signed-off-by: wangli <wangli858794774@gmail.com>

benchmarks/scripts/run-performance-benchmarks.sh

@@ -78,7 +78,9 @@ kill_npu_processes() {
   ps -aux
   lsof -t -i:8000 | xargs -r kill -9
   pgrep python3 | xargs -r kill -9
-  
+  # vLLM now names the process with VLLM prefix after https://github.com/vllm-project/vllm/pull/21445
+  pgrep VLLM | xargs -r kill -9
+
   sleep 4
   rm -rf ~/.config/vllm