container: always check if mountpoint is mounted

when running as a service, the c.state.Mounted flag could get out of
sync if the container is cleaned up through the cleanup process.

To avoid this, always check if the mountpoint is really present before
skipping the mount.

[NO NEW TESTS NEEDED]

Closes: containers#17042

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

libpod/container_internal.go

@@ -1539,7 +1539,11 @@ func (c *Container) mountStorage() (_ string, deferredErr error) {
 	var err error
 	// Container already mounted, nothing to do
 	if c.state.Mounted {
-		return c.state.Mountpoint, nil
+		if mounted, err := mount.Mounted(c.state.Mountpoint); err != nil {
+			return "", fmt.Errorf("unable to determine if %q is mounted: %w", c.state.Mountpoint, err)
+		} else if mounted {
+			return c.state.Mountpoint, nil
+		}
 	}
 
 	if !c.config.NoShm {