optimize mkdir calls to avoid base-dir Mkdir attempts (#18021)

Currently we have IOPs of these patterns

```
[OS] os.Mkdir play.min.io:9000 /disk1 2.718µs
[OS] os.Mkdir play.min.io:9000 /disk1/data 2.406µs
[OS] os.Mkdir play.min.io:9000 /disk1/data/.minio.sys 4.068µs
[OS] os.Mkdir play.min.io:9000 /disk1/data/.minio.sys/tmp 2.843µs
[OS] os.Mkdir play.min.io:9000 /disk1/data/.minio.sys/tmp/d89c8ceb-f8d1-4cc6-b483-280f87c4719f 20.152µs
```

It can be seen that we can save quite Nx levels such as
if your drive is mounted at `/disk1/minio` you can simply
skip sending an `Mkdir /disk1/` and `Mkdir /disk1/minio`.

Since they are expected to exist already, this PR adds a way
for us to ignore all paths upto the mount or a directory which
ever has been provided to MinIO setup.

cmd/os_unix.go

@@ -24,6 +24,7 @@ import (
 	"bytes"
 	"fmt"
 	"os"
+	"strings"
 	"sync"
 	"syscall"
 	"unsafe"
@@ -47,7 +48,13 @@ func access(name string) error {
 // directories that MkdirAll creates.
 // If path is already a directory, MkdirAll does nothing
 // and returns nil.
-func osMkdirAll(dirPath string, perm os.FileMode) error {
+func osMkdirAll(dirPath string, perm os.FileMode, baseDir string) error {
+	if baseDir != "" {
+		if strings.HasPrefix(baseDir, dirPath) {
+			return nil
+		}
+	}
+
 	// Slow path: make sure parent exists and then call Mkdir for path.
 	i := len(dirPath)
 	for i > 0 && os.IsPathSeparator(dirPath[i-1]) { // Skip trailing path separator.
@@ -61,7 +68,7 @@ func osMkdirAll(dirPath string, perm os.FileMode) error {
 
 	if j > 1 {
 		// Create parent.
-		if err := osMkdirAll(dirPath[:j-1], perm); err != nil {
+		if err := osMkdirAll(dirPath[:j-1], perm, baseDir); err != nil {
 			return err
 		}
 	}