[warm/fast-reboot] Backup logs from tmpfs to disk during fast/warm shutdown (sonic-net#2714)

Goal: Preserve logs during TOR upgrades and shutdown

Need:

Below PRs moved logs from disk to tmpfs for specific hwskus.
Due to these changes, shutdown path logs are now lost.
The logs in shutdown path are crucial for debug purposes.

sonic-net/sonic-buildimage#13805
sonic-net/sonic-buildimage#13587
sonic-net/sonic-buildimage#13587

How I did it
Check if logs are on tmpfs. If yes, backup logs from /var/log

How to verify it
Verified on a physical device - logs on tmfs are backed up for past 30 minutes.

Author: vaibhavhd

scripts/fast-reboot

@@ -806,6 +806,17 @@ fi
 
 # Reboot: explicitly call Linux native reboot under sbin
 debug "Rebooting with ${REBOOT_METHOD} to ${NEXT_SONIC_IMAGE} ..."
+
+LOGS_ON_TMPFS=0
+df --output=fstype /var/log* | grep -c 'tmpfs' || LOGS_ON_TMPFS=$?
+if [[ LOGS_ON_TMPFS -eq 0 ]]; then
+    debug "Backup shutdown logs to /host/logs_before_reboot"
+    mkdir -p /host/logs_before_reboot || /bin/true
+    # maxdepth 2: find files within 2 nested directories (eg. /var/log/ and /var/log/swss/)
+    # mmin 30: find files written in past 30 minutes
+    find /var/log -maxdepth 2 -mmin -30 -type f | xargs -I {} cp {} /host/logs_before_reboot/ || /bin/true
+fi
+
 exec ${REBOOT_METHOD}
 
 # Should never reach here