Fix mount_entry_on_generic()

Fix mount_entry_on_generic()

In mount_entry_on_generic() we dereferenced a NULL pointer whenever a container
without a rootfs was created. (Since mount_entry_on_systemfs() passes them with
NULL.) We have mount_entry_on_generic() check whether rootfs != NULL.

We also check whether rootfs != NULL in the functions ovl_mkdir() and
mount_entry_create_aufs_dirs() and bail immediately. Rationale: For overlay and
aufs lxc.mount.entry entries users give us absolute paths to e.g. workdir and
upperdir which we create for them. We currently use rootfs->path and the
lxcpath for the container to check that users give us a sane path to create
those directories under and refuse if they do not. If we want to allow overlay
mounts for containers without a rootfs they can easily be reworked.

Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>