[buffer] indicate shared pool by putting a very small shared buffer p…

…ool size (sonic-net#3387)

What I did
Put a very small shared buffer pool size when shared pool is detected enabling.

Why I did it
Avoid a timing issue like following:

the buffer pool sizes, xoff have initialized to 0, which means the shared headroom pool is disabled
but the buffer profiles already indicate the shared headroom pool is enabled
later on the buffer pool sizes are updated with xoff being non-zero
In case the orchagent starts handling buffer configuration between 2 and 3, it is inconsistent between buffer pools and profiles, which fails Mellanox SAI sanity check and causes orchagent abort

cfgmgr/buffer_pool_mellanox.lua

@@ -166,7 +166,11 @@ local function fetch_buffer_pool_size_from_appldb(shp_enabled)
                 -- In case the orchagent starts handling buffer configuration between 2 and 3,
                 -- It is inconsistent between buffer pools and profiles, which fails Mellanox SAI sanity check
                 -- To avoid it, it indicates the shared headroom pool is enabled by setting a very small buffer pool and shared headroom pool sizes
-                table.insert(result, buffer_pools[i] .. ':2048:1024')
+                if size == "0" then
+                    table.insert(result, buffer_pools[i] .. ':2048:1024')
+                else
+                    table.insert(result, buffer_pools[i] .. ":" .. size .. ':1024')
+                end
             else
                 table.insert(result, buffer_pools[i] .. ':' .. size)
             end