meta: fix negative values in quota before used in statfs

[SandyXSD]

df may display negative value:

$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/vda2       178G   44G  127G  26% /
JuiceFS:test    1.0P -100K  1.0P   0% /mnt/jfs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.51%. Comparing base (a76b419) to head (b96df26).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4591      +/-   ##
==========================================
- Coverage   55.54%   55.51%   -0.03%     
==========================================
  Files         159      159              
  Lines       42932    42940       +8     
==========================================
- Hits        23846    23839       -7     
- Misses      16422    16439      +17     
+ Partials     2664     2662       -2     

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