Set egress mirror headroom to 0 on SN6600 platform

Signed-off-by: Stephen Sun <[email protected]>

Author: stephenxs

cfgmgr/buffer_pool_mellanox.lua

@@ -199,6 +199,7 @@ if platform then
         -- Reserve 32MB for modification descriptors pool
         if model_number and model_number >= 6000 then
             modification_descriptors_pool_size = 32 * 1024 * 1024
+            egress_mirror_headroom  = 0
         end
     end
 end