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.

internal/ioutil/read_file.go

@@ -20,6 +20,8 @@ package ioutil
 import (
 	"io"
 	"os"
+
+	"github.com/minio/minio/internal/disk"
 )
 
 // ReadFile reads the named file and returns the contents.
@@ -33,6 +35,10 @@ func ReadFile(name string) ([]byte, error) {
 	if err != nil {
 		return nil, err
 	}
+	if err := disk.Fadvise(f, disk.FadvSequential); err != nil {
+		return nil, err
+	}
+	defer disk.Fadvise(f, disk.FadvNoReuse)
 	defer f.Close()
 	st, err := f.Stat()
 	if err != nil {