fix: read metadata in O_DIRECT if configured and supported (#11594)

reduce the page-cache pressure completely by moving
the entire read-phase of our operations to O_DIRECT,
primarily this is going to be very useful for chatty
metadata operations such as listing, scanner, ilm, healing
like operations to avoid filling up the page-cache upon
repeated runs.

cmd/storage-errors.go

@@ -143,7 +143,7 @@ func osErrToFileErr(err error) error {
 	if osIsPermission(err) {
 		return errFileAccessDenied
 	}
-	if isSysErrNotDir(err) {
+	if isSysErrNotDir(err) || isSysErrIsDir(err) {
 		return errFileNotFound
 	}
 	if isSysErrPathNotFound(err) {