xfstests: don't suggest to run full diff when DIFF_LENGTH is 0

If DIFF_LENGTH is set to 0, the full output is available and the message
does not make sense.

Signed-off-by: David Sterba <dsterba@suse.cz>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

check

@@ -510,11 +510,11 @@ do
 				cat
 			else
 				head -n "$DIFF_LENGTH"
+				echo "..."
+				echo "(Run '$diff $seq.out $seqres.out.bad'" \
+					" to see the entire diff)"
 			fi; } | \
 			sed -e 's/^\(.\)/    \1/'
-		    echo "     ..."
-		    echo "     (Run '$diff $seq.out $seqres.out.bad' to see the" \
-			 "entire diff)"
 		    err=true
 		fi
 	    fi