ubi: Free the normal volumes in error paths of ubi_attach_mtd_dev()

The allocated normal volumes saved in ubi->volumes are not freed
in the error paths in ubi_attach_mtd_dev() and its callees (e.g.
ubi_attach() and ubi_read_volume_table()).

These normal volumes should be freed through kill_volumes() and
vol_release(), but ubi_attach_mtd_dev() may fail before
calling uif_init(), and there will be memory leaks.

So adding a new helper ubi_free_all_volumes() to free the normal
and the internal volumes. And in order to prevent double-free
of volume, reset ubi->volumes[i] to NULL after freeing.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>