Avoid ELOOP on auto-mounted snapshots

Ensure that the path member pointers are associated with the
newly-mounted snapshot when zpl_snapdir_automount() returns.  Otherwise
the follow_automount() function may be called repeatedly, leading to an
incorrect ELOOP error return. This problem was observed as a 'Too many
levels of symbolic links' error from user-space commands accessing an
unmounted snapshot in the .zfs/snapshot directory.

Issue openzfs#816

module/zfs/zpl_ctldir.c

@@ -356,6 +356,13 @@ zpl_snapdir_automount(struct path *path)
 	if (error)
 		return ERR_PTR(error);
 
+	/*
+	 * Ensure path->dentry points to the dentry for the root of the
+	 * newly-mounted snapshot, otherwise this function may be called
+	 * repeatedly which can lead to an incorrect ELOOP error return.
+	 */
+	follow_up(path);
+
 	/*
 	 * Rather than returning the new vfsmount for the snapshot we must
 	 * return NULL to indicate a mount collision.  This is done because