overlay: avoid to use NULL OVL_BASE_FSTYP for mounting

overlay: avoid to use NULL OVL_BASE_FSTYP for mounting

Generally, FSTYP is used to specify OVL_BASE_FSTYP. When we specify FSTYP
through an environment variable, it is not converted to OVL_BASE_FSTYP.
In addition, sometimes we do not even specify the file type. For example,
we only use `./check -n -overlay -g auto` to list overlay-related cases.
If OVL_BASE_FSTYP is NULL, mounting fails and the test fails.

To solve this problem, try to assign a value to OVL_BASE_FSTYP when
specifying -overlay. In addition, in the _overlay_base_mount function,
the basic file system type of the overlay is specified only when
OVL_BASE_FSTYP is not NULL.

Reported-by: Murphy Zhou <jencce.kernel@gmail.com>
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>