overlay: hook filesystem check helper

overlay: hook filesystem check helper

Hook filesystem check helper to _check_test_fs and _check_scratch_fs
for checking consistency of underlying dirs of overlay filesystem.
These helpers works only if fsck.overlay exists.

This patch introduce OVERLAY_FSCK_OPTIONS use for check overlayfs like
OVERLAY_MOUNT_OPTIONS, and also introduce a mount point check helper in
common/rc to detect a dir is a mount point or not.

[ _check_test_fs/_check_scratch_fs part picked from Amir's patch, thanks ]

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