Large buckets with millions of objects can take longer than the default
5-second walkdir timeout to produce the first page of listing results.
This causes timeouts in the web UI and mc CLI when opening or scanning
such buckets.
Skip the walkdir total timeout for S3 ListObjects operations when no
explicit walkdir_timeout is configured. The stall timeout (5s with no
forward progress) still protects against drives that stop responding.
This matches the scanner's existing behavior for the same reason.
Fixes#5647
Zhengchao An
1934cddd66