mount.Umount: EBUSY: try at most 50 times

The function is being used in a number of places, notably container
removal and cleanup.  While container removal already loops over EBUSY,
cleanup does not.

To make sure that all callers of Unmount get a fair chance of unmounting
cleanly, also loop there.  I used the same values as containerd: 50
loops with 50ms sleeps.

Context: containers/podman/issues/11594
Signed-off-by: Valentin Rothberg <vrothberg@redhat.com>

pkg/mount/unmount_unix.go

@@ -1,16 +1,30 @@
+//go:build !windows
 // +build !windows
 
 package mount
 
-import "golang.org/x/sys/unix"
+import (
+	"time"
+
+	"golang.org/x/sys/unix"
+)
 
 func unmount(target string, flags int) error {
-	err := unix.Unmount(target, flags)
-	if err == nil || err == unix.EINVAL {
-		// Ignore "not mounted" error here. Note the same error
-		// can be returned if flags are invalid, so this code
-		// assumes that the flags value is always correct.
-		return nil
+	var err error
+	for i := 0; i < 50; i++ {
+		err = unix.Unmount(target, flags)
+		switch err {
+		case unix.EBUSY:
+			time.Sleep(50 * time.Millisecond)
+			continue
+		case unix.EINVAL, nil:
+			// Ignore "not mounted" error here. Note the same error
+			// can be returned if flags are invalid, so this code
+			// assumes that the flags value is always correct.
+			return nil
+		default:
+			break
+		}
 	}
 
 	return &mountError{