[logrotate] Fix logrotate failure when asic.conf absent.

[puffc]

fixes #15932

Why I did it

The current logrotate postrotate script can be able to rotate swss.rec and sairedis.rec for multiasic platform. However, for single asic platform, the postrotate script doesn't work due to $NUM_ASIC is unset.

Work item tracking

• Microsoft ADO (number only):

How I did it

Use brace expansion to set default value 1 if NUM_ASIC is null or unset. Please note that the postrotate script will be executed by posix shell only.

How to verify it

logrotate manually:

admin@sonic:~$ ls /var/log/swss -la
total 16
drwxr-xr-x 2 root root 4096 Jul 21 14:56 .
drwxr-xr-x 7 root root 4096 Jul 21 14:54 ..
-rw-r--r-- 1 root root  831 Jul 21 14:57 sairedis.rec
-rw-r--r-- 1 root root   45 Jul 21 14:57 swss.rec
admin@sonic:~$ sudo /usr/sbin/logrotate -f /etc/logrotate.conf
admin@sonic:~$ ls /var/log/swss -la
total 396
drwxr-xr-x 2 root root   4096 Jul 21 14:58 .
drwxr-xr-x 7 root root   4096 Jul 21 14:58 ..
-rw-r--r-- 1 root root  17743 Jul 21 14:58 sairedis.rec
-rw-r--r-- 1 root root 331513 Jul 21 14:58 sairedis.rec.1
-rw-r--r-- 1 root root      0 Jul 21 14:58 swss.rec
-rw-r--r-- 1 root root  41857 Jul 21 14:57 swss.rec.1
admin@sonic:~$ sudo cat /var/log/syslog|grep logrotate
Jul 21 14:58:03.481929 sonic INFO logrotate: Sending SIGHUP to OA log_file_name: /var/log/swss/sairedis.rec
Jul 21 14:58:04.673242 sonic INFO logrotate: Sending SIGHUP to OA log_file_name: /var/log/swss/swss.rec

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205
• 202211
• 202305

Tested branch (Please provide the tested image version)

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[puffc]

@lguohan @arlakshm @prgeor Could you please review/merge this PR?

[saiarcot895]

Looks like this has been working so far because -gt 1 (when asic.conf is missing and NUM_ASIC is undefined) is an error, but is treated as false for the purposes of the if-condition, and it goes into the single ASIC path, but yes, this should be fixed.

[puffc]

@qiluo-msft Need your help to review/merge this PR. Thanks!

[puffc]

@qiluo-msft Need your help to review/merge this PR. Thanks!

[puffc]

@qiluo-msft Need your help to review/merge this PR. Thanks!

[puffc]

@lguohan Would you please review this PR?