curvefs/client: diskcache division by zero

[h0hmj]

Describe the bug (描述bug)

curve/curvefs/src/client/s3/disk_cache_manager.cpp

Line 263 in bc036ee

int64_t DiskCacheManager::SetDiskFsUsedRatio() {

using statfs to get info, but sometime usedBytes and availableBytes will be zero both, cause division by zero.
int64_t usedPercent = 100 * usedBytes / (usedBytes + availableBytes) + 1;

some systems do not implement statfs, so data may be incorrect.

To Reproduce (复现方法)

using 9p mount local dir to minikube (minikube mount), then run curve-fuse with diskcache enable in minikube, will core dump because of division by zero

related issue
kubernetes/minikube#3794

Expected behavior (期望行为)

No Coredump but Error Log and exit correctly