should not compare checksum for symlink when sync.

[zhoucheng361]

root@bench-01:~/jfs# ls jfs_source/pkg/symlink_to_cmd -l
lrwxrwxrwx 1 root root 6 Sep 25 16:26 jfs_source/pkg/symlink_to_cmd -> ../cmd

root@bench-01:~/jfs# cmd/mount/mount sync jfs_source/ /tmp/sync1/ --threads 1 --links --limit 740 --bwlimit 0 --check-all
2023/09/25 16:35:10.175823 juicefs[1832075] <INFO>: Syncing from file:///root/jfs/jfs_source/ to file:///tmp/sync1/ [sync.go:1043]
2023/09/25 16:35:15.212099 juicefs[1832075] <ERROR>: Failed to check pkg/symlink_to_cmd in 5.001413756s: src read: EOF [sync.go:320]
Scanned objects: 740/740 [==============================================================]  146.9/s used: 5.036789912s
Skipped objects: 739                         146.7/s      
Pending objects: 0                           0.0/s        
 Copied objects: 0                           0.0/s        
   Copied bytes: 0.0 b    (0 Bytes)          0.0 b/s      
Checked objects: 739                         146.7/s      
  Checked bytes: 68.1 MiB (71424933 Bytes)   13.5 MiB/s   
 Failed objects: 1                           0.2/s        
2023/09/25 16:35:15.212594 juicefs[1832075] <INFO>: Found: 740, skipped: 739, copied: 0 (0 B), checked: 739 (68.12 MiB), failed: 1 [sync.go:1082]
2023/09/25 16:35:15.212639 juicefs[1832075] <FATAL>: Failed to handle 1 objects [main.go:40]