[fast-reboot] Backup database after syncd/swss stopped (sonic-net#3342)

- What I did
Backup DB after syncd and swss are stopped. I observed an issue with fast-reboot that in a rare circumstances a queued FDB event might be written to ASIC_DB by a thread inside syncd after a call to FLUSHDB ASIC_DB was made.
That left ASIC_DB only with one record about that FDB entry and caused syncd to crash at start:

Mar 15 13:28:42.765108 sonic NOTICE syncd#SAI: :- Syncd: syncd started
Mar 15 13:28:42.765268 sonic NOTICE syncd#SAI: :- onSyncdStart: performing hard reinit since COLD start was performed
Mar 15 13:28:42.765451 sonic NOTICE syncd#SAI: :- readAsicState: loaded 1 switches
Mar 15 13:28:42.765465 sonic NOTICE syncd#SAI: :- readAsicState: switch VID: oid:0x21000000000000
Mar 15 13:28:42.765465 sonic NOTICE syncd#SAI: :- readAsicState: read asic state took 0.000205 sec
Mar 15 13:28:42.766364 sonic NOTICE syncd#SAI: :- onSyncdStart: on syncd start took 0.001097 sec
Mar 15 13:28:42.766376 sonic ERR syncd#SAI: :- run: Runtime error during syncd init: map::at
Mar 15 13:28:42.766376 sonic NOTICE syncd#SAI: :- sendShutdownRequest: sending switch_shutdown_request notification to OA for switch: oid:0x0
Mar 15 13:28:42.766518 sonic NOTICE syncd#SAI: :- sendShutdownRequestAfterException: notification send successfully

- How I did it
Backup DB after syncd/swss have stopped.

- How to verify it
Run fast-reboot.

Signed-off-by: Stepan Blyschak <stepanb@nvidia.com>

scripts/fast-reboot

@@ -244,6 +244,19 @@ function wait_for_pre_shutdown_complete_or_fail()
 function backup_database()
 {
     debug "Backing up database ..."
+
+    if [[ "$REBOOT_TYPE" = "fastfast-reboot" || "$REBOOT_TYPE" = "fast-reboot" ]]; then
+        # Advanced reboot: dump state to host disk
+        sonic-db-cli ASIC_DB FLUSHDB > /dev/null
+        sonic-db-cli COUNTERS_DB FLUSHDB > /dev/null
+        sonic-db-cli FLEX_COUNTER_DB FLUSHDB > /dev/null
+    fi
+
+    if [[ "$REBOOT_TYPE" = "fast-reboot" ]]; then
+        # Flush RESTAP_DB in fast-reboot to avoid stale status
+        sonic-db-cli RESTAPI_DB FLUSHDB > /dev/null
+    fi
+
     # Dump redis content to a file 'dump.rdb' in warmboot directory
     mkdir -p $WARM_DIR
     # Delete keys in stateDB except FDB_TABLE|*, MIRROR_SESSION_TABLE|*, WARM_RESTART_ENABLE_TABLE|*, FG_ROUTE_TABLE|*
@@ -806,23 +819,11 @@ for service in ${SERVICES_TO_STOP}; do
             wait_for_pre_shutdown_complete_or_fail
         fi
 
-        if [[ "$REBOOT_TYPE" = "fastfast-reboot" || "$REBOOT_TYPE" = "fast-reboot" ]]; then
-            # Advanced reboot: dump state to host disk
-            sonic-db-cli ASIC_DB FLUSHDB > /dev/null
-            sonic-db-cli COUNTERS_DB FLUSHDB > /dev/null
-            sonic-db-cli FLEX_COUNTER_DB FLUSHDB > /dev/null
-        fi
-
-        if [[ "$REBOOT_TYPE" = "fast-reboot" ]]; then
-            # Flush RESTAP_DB in fast-reboot to avoid stale status
-            sonic-db-cli RESTAPI_DB FLUSHDB > /dev/null
-        fi
-
-        backup_database
-
     fi
 done
 
+backup_database
+
 # Stop the docker container engine. Otherwise we will have a broken docker storage
 systemctl stop docker.service || debug "Ignore stopping docker service error $?"