meta: remove tail zero slice in compactChunk

[SandyXSD]

$ dd if=/dev/zero of=/jfs/tmp bs=1M count=10
$ truncate -s 4 /jfs/tmp
$ juicefs compact /jfs/tmp

# before
$ juicefs info /jfs/tmp
 objects:
+------------+----------------------------------+---------+--------+---------+
| chunkIndex |            objectName            |   size  | offset |  length |
+------------+----------------------------------+---------+--------+---------+
|          0 | test/chunks/0/24/24578_0_4194304 | 4194304 |      0 | 4194304 |
|          0 | test/chunks/0/24/24578_1_4194304 | 4194304 |      0 | 4194304 |
|          0 | test/chunks/0/24/24578_2_2097152 | 2097152 |      0 | 2097152 |
+------------+----------------------------------+---------+--------+---------+

# with this fix
$ juicefs info /jfs/tmp
 objects:
+------------+----------------------------+------+--------+--------+
| chunkIndex |         objectName         | size | offset | length |
+------------+----------------------------+------+--------+--------+
|          0 | test/chunks/0/28/28674_0_4 |    4 |      0 |      4 |
+------------+----------------------------+------+--------+--------+