posix: cache disk ID for a short while (#8564)
posix: cache disk ID for a short while (#8564)

`*posix.getDiskID()` takes up to 30% of all CPU due to the `os.Stat` call on `GET` calls.

Before:
```
Operation: GET - Concurrency: 12
* Average: 1333.97 MB/s, 1365.99 obj/s, 1365.98 ops ended/s (4m59.975s)
* First Byte: Average: 7.801487ms, Median: 7.9974ms, Best: 1.9822ms, Worst: 110.0021ms

Aggregated, split into 299 x 1s time segments:
* Fastest: 1453.50 MB/s, 1488.38 obj/s, 1492.00 ops ended/s (1s)
* 50% Median: 1360.47 MB/s, 1393.12 obj/s, 1393.00 ops ended/s (1s)
* Slowest: 978.68 MB/s, 1002.17 obj/s, 1004.00 ops ended/s (1s)
```

After:
```
Operation: GET - Concurrency: 12
* Average: 1706.07 MB/s, 1747.02 obj/s, 1747.01 ops ended/s (4m59.985s)
* First Byte: Average: 5.797886ms, Median: 5.9959ms, Best: 996.3µs, Worst: 84.0007ms

Aggregated, split into 299 x 1s time segments:
* Fastest: 1830.03 MB/s, 1873.96 obj/s, 1872.00 ops ended/s (1s)
* 50% Median: 1735.04 MB/s, 1776.68 obj/s, 1776.00 ops ended/s (1s)
* Slowest: 994.94 MB/s, 1018.82 obj/s, 1018.00 ops ended/s (1s)
```

TLDR; `os.Stat` is not free.
harshavardhana/minio,aead/minio,krisis/minio,minio/minio,aead/minio,harshavardhana/minio,vadmeste/minio,harshavardhana/minio,donatello/minio,vadmeste/minio,donatello/minio,aead/minio,balamurugana/minio,NitishT/minio,NitishT/minio,fwessels/minio,balamurugana/minio,aead/minio,minio/minio,aead/minio,harshavardhana/minio,fwessels/minio,harshavardhana/minio,krisis/minio,aead/minio,harshavardhana/minio,aead/minio,harshavardhana/minio,aead/minio,fwessels/minio,NitishT/minio,fwessels/minio,NitishT/minio,harshavardhana/minio