store: retry RemoveAll on EBUSY

when running on NFS, a RemoveAll could cause EBUSY because of some
unlinked files that are still kept open and "silly renamed" to
.nfs$ID.

These files could be kept open by conmon and the issue is addressed
by: containers/conmon#300

Before failing, attempt a few more times.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

store.go

@@ -28,6 +28,7 @@ import (
 	digest "github.com/opencontainers/go-digest"
 	"github.com/opencontainers/selinux/go-selinux/label"
 	"github.com/pkg/errors"
+	"golang.org/x/sys/unix"
 )
 
 var (
@@ -2498,7 +2499,15 @@ func (s *store) DeleteContainer(id string) error {
 			gcpath := filepath.Join(s.GraphRoot(), middleDir, container.ID)
 			wg.Add(1)
 			go func() {
-				errChan <- os.RemoveAll(gcpath)
+				var err error
+				for attempts := 0; attempts < 50; attempts++ {
+					err = os.RemoveAll(gcpath)
+					if err == nil || !errors.Is(err, unix.EBUSY) {
+						break
+					}
+					time.Sleep(time.Millisecond * 100)
+				}
+				errChan <- err
 				wg.Done()
 			}()