overlayfs: improve wrapper lifetime management with root refcount

overlayfs: improve wrapper lifetime management with root refcount

Implement M6: overlayfs wrapper lifetime improvements.

Added is_root flag to overlay_node to distinguish root from wrapper nodes.
Modified overlay_root_close() to properly manage root node refcount and
free overlayfs structure when root is finally closed.
Initialized root node refcount to 1 and set is_root flag in overlayfs_create_root().

This ensures proper cleanup of overlayfs structures when the filesystem
is unmounted, preventing memory leaks of the overlayfs structure and root node.

Test: make test-battery PASS (157/157), make analyzer PASS