msdosfs: handle a case when non-dot lookup returned dvp

This means that filesystem is corrupted, there is a loop.

In collaboration with:	pho
Reviewed by:	markj, mckusick
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D33721

sys/fs/msdosfs/msdosfs_lookup.c

@@ -573,6 +573,10 @@ foundroot:
 			goto restart;
 		}
 	} else if (dp->de_StartCluster == scn && isadir) {
+		if (cnp->cn_namelen != 1 || cnp->cn_nameptr[0] != '.') {
+			/* fs is corrupted, non-dot lookup returned dvp */
+			return (EBADF);
+		}
 		VREF(vdp);	/* we want ourself, ie "." */
 		*vpp = vdp;
 	} else {