use fadvise to control Linux page-cache (#13312)

This PR brings two optimizations mainly
for page-cache build-up and how to avoid
getting OOM killed in the process. Although
these memories are reclaimable Linux is not
fast enough to reclaim them as needed on a
very busy system. fadvise is a system call
implemented in Linux to advise page-cache to
avoid overload as we get significant amount
of requests on the server.

- FADV_SEQUENTIAL tells that all I/O from now
  is going to be sequential, allowing for more
  resposive throughput.

- FADV_NOREUSE tells kernel to start removing
  things for this 'fd' from page-cache.

cmd/os-readdir_unix.go

@@ -28,6 +28,7 @@ import (
 	"syscall"
 	"unsafe"
 
+	"github.com/minio/minio/internal/disk"
 	"golang.org/x/sys/unix"
 )
 
@@ -110,6 +111,11 @@ func readDirFn(dirPath string, fn func(name string, typ os.FileMode) error) erro
 		}
 		return osErrToFileErr(err)
 	}
+	if err := disk.Fadvise(f, disk.FadvSequential); err != nil {
+		return err
+	}
+
+	defer disk.Fadvise(f, disk.FadvNoReuse)
 	defer f.Close()
 
 	bufp := direntPool.Get().(*[]byte)
@@ -185,6 +191,12 @@ func readDirWithOpts(dirPath string, opts readDirOpts) (entries []string, err er
 	if err != nil {
 		return nil, osErrToFileErr(err)
 	}
+
+	if err := disk.Fadvise(f, disk.FadvSequential); err != nil {
+		return nil, err
+	}
+
+	defer disk.Fadvise(f, disk.FadvNoReuse)
 	defer f.Close()
 
 	bufp := direntPool.Get().(*[]byte)