Fix a problem when you try to mount a directory on another directory

belongs to the same filesystem. In this problem, getcwd(3) will fail.

I found the problem two years ago and I have forgotten to merge.

http://docs.FreeBSD.org/cgi/mid.cgi?200202251435.XAA91094

sys/fs/unionfs/union_vnops.c

@@ -972,6 +972,8 @@ union_getattr(ap)
 		union_newsize(ap->a_vp, VNOVAL, vap->va_size);
 	}
 
+	ap->a_vap->va_fsid = ap->a_vp->v_mount->mnt_stat.f_fsid.val[0];
+
 	if ((vap != ap->a_vap) && (vap->va_type == VDIR))
 		ap->a_vap->va_nlink += vap->va_nlink;
 	return (0);