fix error: interface counters is mismatch after warm-reboot (sonic-ne…

…t#1099)

- What I did
There is a issue for counters after warm-reboot:
If I clear counters by command "sonic-clear counters", then execute 'warm-reboot' and whenSONiC is restart, the counters showed with command "show interface counters" is still old counters before "sonic-clear". It is not the right counters because the counters file in '/tmp' is lost in warm-reboot process.

- How I did it
I fixed it by saving '/tmp/portstat-0' folders in '/host/' before executing 'warm-reboot', and restore the counters folders back to '/tmp/' after warm-reboot process is finished (in pull request sonic-net/sonic-buildimage#5346 ).

- How to verify it
Clear counters by command 'sonic-clear'
sonic-clear counters
sonic-clear dropcounters
sonic-clear pfccounters
sonic-clear queuecounters
sonic-clear rifcounters
Execute 'warm-reboot'
Use command ‘show interface counters’ to see if the counters is right.

scripts/fast-reboot

@@ -362,6 +362,25 @@ function unload_kernel()
     fi
 }
 
+function save_counters_folder() {
+    debug "Saving counters folder before warmboot..."
+
+    counters_folder="/host/counters"
+    if [[ ! -d $counters_folder ]]; then
+        mkdir $counters_folder
+    fi
+    if [[ "$REBOOT_TYPE" = "warm-reboot" || "$REBOOT_TYPE" = "fastfast-reboot" ]]; then
+        modules=("portstat-0" "dropstat" "pfcstat-0" "queuestat-0" "intfstat-0")
+        for module in ${modules[@]}
+        do
+            statfile="/tmp/$module"
+            if [[ -d $statfile ]]; then
+                cp -rf $statfile $counters_folder
+            fi
+        done
+    fi
+}
+
 # main starts here
 parseOptions $@
 
@@ -401,6 +420,8 @@ case "$REBOOT_TYPE" in
         ;;
 esac
 
+save_counters_folder
+
 unload_kernel
 
 setup_reboot_variables