Generalize use of Vnode::covered_by/covers

* Introduce Vnode flags for covered and covering. Can be used as a quick
  check when one doesn't already hold sVnodeLock.
* Rename resolve_mount_point_to_volume_root() to
  resolve_vnode_to_covering_vnode().
* Adjust all code that deals with transitions between mount points and
  volume root vnodes to generally support covered/covering vnodes.

headers/private/kernel/vfs.h

@@ -143,7 +143,7 @@ status_t	vfs_create_special_node(const char *path, fs_vnode *subVnode,
 				struct vnode **_createdVnode);
 
 /* service call for the node monitor */
-status_t	resolve_mount_point_to_volume_root(dev_t mountID, ino_t nodeID,
+status_t	resolve_vnode_to_covering_vnode(dev_t mountID, ino_t nodeID,
 				dev_t *resolvedMountID, ino_t *resolvedNodeID);
 
 /* calls the syscall dispatcher should use for user file I/O */