Bug 2148853

Description of problem:
This bug comes from an upstream issue and can be reproduced downstream.
https://gitlab.com/virtio-fs/virtiofsd/-/issues/67

'''
When using python and writing to a mmap file, I am seeing with cache mode auto and always that the file is increasing in size each time I write to it.
'''

Version-Release number of selected component (if applicable):
qemu-kvm-7.1.0-3.el9.x86_64
virtiofsd-1.4.0-1.el9.x86_64
kernel-5.14.0-178.el9.x86_64

How reproducible:
100%

Steps to Reproduce:
1. start virtiofsd
# /usr/libexec/virtiofsd -o source=/home/test  --socket-path=/tmp/sock1 --cache auto
2. boot up a rhel920 guest
3. in guest, mount virtiofs
# mount -t virtiofs myfs /mnt/test
4. write to the same space of a file with mmap 
```

[root@vm-74-75 test]# cat mmap_test.py 
import mmap
import struct
l = 1024
f = open("./test.db", "a+b")
f.truncate(l)
m = mmap.mmap(f.fileno(), 1024, access=mmap.ACCESS_WRITE)
m[0:4] = b"0000"
m.flush()
print(m.size())
m[0:4] = b"0000"
m.flush()
print(m.size())

```

Actual results:
# python3 mmap_test.py 
2048
4096


Expected results:
The file size will not be changed.

Additional info: