[bugfix] A2 Environment Pooling for Memcache Compatibility (#5601)

### What this PR does / why we need it?
When running memcache in the A2 environment, the logic for registering
memory needs to be added. Additionally, there is a link establishment
conflict between memcache and HCCS during initialization in A2, so the
link should be established in advance.

### Does this PR introduce _any_ user-facing change?

### How was this patch tested?

- vLLM version: v0.13.0
- vLLM main:
7157596103

---------

Signed-off-by: fangjianwei <f30058701@china.huawei.com>
Co-authored-by: fangjianwei <f30058701@china.huawei.com>

vllm_ascend/distributed/kvpool/pool_scheduler.py

@@ -82,7 +82,10 @@ class KVPoolScheduler:
         if num_external_hit_tokens == request.num_tokens:
             num_external_hit_tokens -= 1
 
-        need_to_allocate = num_external_hit_tokens - num_computed_tokens
+        if num_external_hit_tokens < num_computed_tokens:
+            need_to_allocate = 0
+        else:
+            need_to_allocate = num_external_hit_tokens - num_computed_tokens
 
         logger.info(
             "Reqid: %s, Total tokens %d, kvpool hit tokens: %d, need to load: %d",