RUSTFS_CAPACITY_MAX_FILES_THRESHOLD=0 made every file count as
overflow with an empty exact prefix, so disks with fewer files than the
sample rate committed 0 bytes as the cluster total; STAT_TIMEOUT=0 with
dynamic timeout disabled made every scan fail at the first entry. Both
were accepted unvalidated, unlike the already-clamped sample_rate and
interval values.
Zero values for the threshold and the stat/min/max timeouts now fall
back to their defaults with a single structured warn at config load
(S11). The symlink-depth knob is left untouched here: it is removed
entirely by the backlog#1018 fix.
Ref: rustfs/backlog#1019 (S11 from audit rustfs/backlog#1010)
