umount: wait for writting staging chunks

[Hexilee]

Signed-off-by: xixi [email protected]

close #2644

Example

hexi@ArchLinux ~/s/juicefs (main)> sudo ./juicefs umount ~/mnt/jfs/
6.6 GiB staging chunks are being written back... 126 MiB/s, left 53s

[codecov-commenter]

Codecov Report

Base: 55.68% // Head: 55.57% // Decreases project coverage by -0.12% ⚠️

Coverage data is based on head (6832a60) compared to base (c8ea744).
Patch coverage: 6.66% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3224      +/-   ##
==========================================
- Coverage   55.68%   55.57%   -0.12%     
==========================================
  Files         142      142              
  Lines       33056    33140      +84     
==========================================
+ Hits        18408    18417       +9     
- Misses      12706    12779      +73     
- Partials     1942     1944       +2     

Impacted Files	Coverage Δ
cmd/umount.go	26.51% <6.66%> (-27.73%)	⬇️
pkg/vfs/fill.go	65.21% <0.00%> (-2.46%)	⬇️
pkg/chunk/cached_store.go	72.77% <0.00%> (-0.14%)	⬇️
pkg/meta/tkv.go	70.82% <0.00%> (-0.13%)	⬇️
pkg/chunk/disk_cache.go	73.38% <0.00%> (ø)
pkg/meta/sql.go	59.06% <0.00%> (+0.14%)	⬆️
pkg/utils/progress.go	76.27% <0.00%> (+1.27%)	⬆️

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