ovl: improve mount helpers
ovl: improve mount helpers

Move common checks into ovl_mount_dir() helper.

Create helper for looking up lower directories.

Signed-off-by: Miklos Szeredi <84cd5483a2f9acd1c37251cba7c2908448f221b8@suse.cz>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs