The length of the file written by the mount point is not updated

[YunhuiChen]

Describe the bug (描述bug)

The length of the file written by the mount point is not updated

To Reproduce (复现方法)
1、mount fs1 -> test1 fs1 -> test2
2、echo a > test1/a
3、cat test2/a -> a
4、echo b >> test1/a
sleep 5mins
5、cat test2/a -> a
6、umount test5 test6 / mount test5 test6

Expected behavior (期望行为)

Versions (各种版本)
OS:
Compiler:
branch:
commit id:

Additional context/screenshots (更多上下文/截图)

[wu-hanqing]

here is a workflow that can reproduce this problem,

           mount_1        │     mount_2
──────────────────────────┼───────────────────────────
                          │
    1. echo "a" > file    │
                          │
    2. cat file           │
       > a                │
                          │   3. cat file
                          │      > a
                          │
    4. echo "a" >> file   │
                          │
                          │
                          │   5. cat file
                          │      > a
                          │
    6. cat file           │
       > a                │
         a                │
                          │   7. cat file
                          │      > a
                          │
    8. umount & remount   │
                          │
                          │
    9. cat file           │
       > a                │

step 9 should output double a like step 6, but it doesn't, because file's length is still 2 instead of 4.

The reason is:

1. we mount a filesystem twice but disable close-to-open guarantee
2. cat file makes the inode dirty in the cache, which means it will update metadata to metaserver, and this leads to the inode's length changing from 4 to 2 in metaserver.