fix: fork os.MkdirAll to optimize cases where parent exists (#15379)

a/b/c/d/ where `a/b/c/` exists results in additional syscalls
such as an Lstat() call to verify if the `a/b/c/` exists
and its a directory.

We do not need to do this on MinIO since the parent prefixes
if exist, we can simply return success without spending
additional syscalls.

Also this implementation attempts to simply use Access() calls
to avoid os.Stat() calls since the latter does memory allocation
for things we do not need to use.

Access() is simpler since we have a predictable structure on
the backend and we know exactly how our path structures are.

cmd/os-instrumented.go

@@ -35,6 +35,7 @@ type osMetric uint8
 const (
 	osMetricRemoveAll osMetric = iota
 	osMetricMkdirAll
+	osMetricMkdir
 	osMetricRename
 	osMetricOpenFile
 	osMetricOpen
@@ -114,10 +115,16 @@ func RemoveAll(dirPath string) error {
 	return os.RemoveAll(dirPath)
 }
 
+// Mkdir captures time taken to call os.Mkdir
+func Mkdir(dirPath string, mode os.FileMode) error {
+	defer updateOSMetrics(osMetricMkdir, dirPath)()
+	return os.Mkdir(dirPath, mode)
+}
+
 // MkdirAll captures time taken to call os.MkdirAll
 func MkdirAll(dirPath string, mode os.FileMode) error {
 	defer updateOSMetrics(osMetricMkdirAll, dirPath)()
-	return os.MkdirAll(dirPath, mode)
+	return osMkdirAll(dirPath, mode)
 }
 
 // Rename captures time taken to call os.Rename