overlay: correct scratch dirs check

overlay: correct scratch dirs check

Tests that use _overlay_scratch_mount_dirs instead of _scratch_mount
should use _require_scratch_nocheck instead of _require_scratch
because these tests are either mounting with multiple lower dirs or
mounting with non-default lower/upper/work dir, so
_check_overlay_scratch_fs won't handle these cases correctly. So we
introduce _overlay_check_scratch_dirs helper and should call this
helper with the correct dir arguments for these non-default cases.

This patch modify these tests to optionally call
_overlay_check_scratch_dirs at the end of the test or after
_scratch_umount to mount base filesystem only and run the checker.

Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>