overlay/075: add test coverage for clearing immutable/append-only flags

overlay/075: add test coverage for clearing immutable/append-only flags

overlay/075 is a variant of check -overlay generic/079.

check -overlay generic/079 does the following operations on overlay fs:
1. Create files
2. Set immutable/append-only flags
3. Verify files/dirs behaving as immutable/append-only
4. Clear immutable/append-only flags
5. Remove files

overlay/075 performs steps 1,2,4,5 on upper and lower layers
and only step 3 is performed on overlay fs (before and after copy up
and mount cycle).

Add also steps 4,5 to be performed on overlay fs to increase the
test coverage of the "merged" inode xflags feature.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>