The tmpfs_link() must not dereference the filesystem-specific data for

a vnode until it is verified that the vnode indeed belongs to tmpfs
mount.  Otherwise, it might access random memory, at least in the
debug kernel.

Reported and tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks

sys/fs/tmpfs/tmpfs_vnops.c

@@ -570,8 +570,6 @@ tmpfs_link(struct vop_link_args *v)
 	MPASS(cnp->cn_flags & HASBUF);
 	MPASS(dvp != vp); /* XXX When can this be false? */
 
-	node = VP_TO_TMPFS_NODE(vp);
-
 	/* XXX: Why aren't the following two tests done by the caller? */
 
 	/* Hard links of directories are forbidden. */
@@ -586,6 +584,8 @@ tmpfs_link(struct vop_link_args *v)
 		goto out;
 	}
 
+	node = VP_TO_TMPFS_NODE(vp);
+
 	/* Ensure that we do not overflow the maximum number of links imposed
 	 * by the system. */
 	MPASS(node->tn_links <= LINK_MAX);