vfs/backup: reduce the limit for skipping backup

[SandyXSD]

1. if backup-meta <= one hour, skip backup when number of inodes exceeds 1m.
2. if backup-meta > one hour, it means user has changed this option explicitly. In this case no more check is applied.

As a reference, a simple test with Redis as meta shows that it takes ~1 mins to backup 1m files, consuming ~1G memory.

[codecov-commenter]

Codecov Report

Merging #1659 (6f5ff52) into main (1e49f93) will decrease coverage by 0.02%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##             main    #1659      +/-   ##
==========================================
- Coverage   61.53%   61.51%   -0.03%     
==========================================
  Files         133      133              
  Lines       22595    22596       +1     
==========================================
- Hits        13904    13900       -4     
- Misses       7126     7128       +2     
- Partials     1565     1568       +3     

Impacted Files	Coverage Δ
pkg/vfs/backup.go	65.97% <42.85%> (+0.35%)	⬆️
pkg/chunk/cached_store.go	75.46% <0.00%> (-0.52%)	⬇️
pkg/meta/sql.go	67.08% <0.00%> (-0.42%)	⬇️
pkg/vfs/reader.go	80.29% <0.00%> (+0.54%)	⬆️
pkg/vfs/writer.go	79.66% <0.00%> (+1.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1e49f93...6f5ff52. Read the comment docs.