[Attention][Bugfix] Fix FA sink support (#28660)

Signed-off-by: Matthew Bonanni <mbonanni@redhat.com>

vllm/v1/attention/backends/flash_attn.py

@@ -130,6 +130,12 @@ class FlashAttentionBackend(AttentionBackend):
             return flash_attn_supports_fp8()
         return kv_cache_dtype in ["auto"]
 
+    @classmethod
+    def supports_sink(cls) -> bool:
+        if not is_flash_attn_varlen_func_available():
+            return False
+        return flash_attn_supports_sinks()
+
     @classmethod
     def supports_compute_capability(cls, capability: DeviceCapability) -> bool:
         return capability >= DeviceCapability(8, 0)